what goes up a hill with three legs and goes down a hill with four legs?

Answers

Answer 1

Answer:

i don't know what goes up with three leg and goes down with 4

Answer 2

Answer:

u

Step-by-step explanation:


Related Questions

What is the median of the data set? {5, 3, 4, 2, 6} Enter your answer in the box. Round to the nearest tenth.

The best answer will receive brainiest.

Answers

Answer:

4

Step-by-step explanation:

To find median, arrange the data set in ascending order. Then the middle term is the median

2 , 3 , 4 , 5 , 6

Median = 3rd term  = 4

Answer:

4 is the median

Step-by-step explanation:

From least to greatest:

2,3,4,5,6

Since the total values is odd, the middle number is the median

4 is the median

Please hurryyyyy !!!! What's the slope and y intercept of 4y = 2x - 12

Answers

Answer:

slop is 0.5 and intercept of y is -3

Answer:

½ is the slope,-3 is the intercept

Step-by-step explanation:

now to find the slope and intercept

we use the equation,

y=mx + c

where m is the slope

and c is the y intercept

so we compare 4y=2x-12 to the equation

but we first have to make y the subject of the equation in the question – 4y is already the subject –

y=2x-12/4

y=2x/4 -3

comparing...

we see that m is 2/4=½ c= -3

17. a type of bacteria doubles in population every 2 hours. given that there were approximately 5 bacteria to start with, how many bacteria will there be in 10 hours?

Answers

There will be approximately 320 bacteria in 10 hours.

To determine the number of bacteria in 10 hours, we need to calculate the number of doubling cycles that occur within that time period.

Since the bacteria population doubles every 2 hours, in 10 hours, there will be 10/2 = 5 doubling cycles.

Starting with approximately 5 bacteria, each doubling cycle will result in the population doubling. Therefore, the number of bacteria after 5 doubling cycles can be calculated as follows

5 bacteria * (2^5) = 5 * 32 = 160

Hence, there will be approximately 160 bacteria after 5 doubling cycles or after 10 hours.

To learn more about doubles

brainly.com/question/31929070

#SPJ11

In desperate need of help. If anyone could help it would be greatly appreciate. Will rate brainliest!

Answers

Answer:

7 people

Step-by-step explanation:

Chocolate: 350 / 100 = 3.5 x 2 = 7

Butter: 45 / 10 = 4.5 x 2 = 9

Eggs: 8 / 2 = 4 x 2 = 8

The amount of chocolate is the limiting factor as you have enough eggs for 8 people and enough butter for 9 people, but only enough chocolate for 7 people

At a school play, an elementary school is accepting donations for a toy drive. An average donation of $8 per person is anticipated. The cost of the set design and costumes is $750, advertising in a local newspaper is $125, and posters to announce the concert cost is $89.

HELP HELP MEEEEE !!!!!

Answers

Answer:

sorry

Step-by-step explanation:

Can someone help me with this. Will Mark brainliest. Need answer and explanation/work. Thank you.

Answers

Answer:

Parallel: y = -2x + 2 (Or any value that isn't 4), Perpendicular: 1/2x + (Anything),

Neither: y = 4x - 7 (Or any random equation that does not have the same, or opposite reciprocal slope)

Step-by-step explanation:

PLSSSS HELPPPP PLSSSS PLS HELPPPPP!!!!!!

Answers

Answer:

B -- (5 smoothings)(price of 1 smoothie) + tip = $23

Step-by-step explanation:

5 smoothies(price of 1 smoothie)+tip=23

How many days in April

Answers

Answer:

There are 30 days according to the Georgian calender of the month of april.

Step-by-step explanation:

Find the cross product a x b where a = (3, -3, -2) and b = (3,-1,-2). ахь Find the cross product cx d where c = (1, -2,-2) and d=(-5,3,-1).

Answers

The cross product of vector c and d is equal to (-4,9,13).

The cross product of two vectors a and b in three-dimensional space is a vector that is perpendicular to both a and b and thus normal to the plane containing them. Here, we have to find the cross product of two given vectors. The given vectors are:a = (3, -3, -2) and b = (3, -1, -2)  So, a x b = (4,0,6).The second one is given as: c = (1, -2,-2) and d=(-5,3,-1).

A vector in mathematics is a quantity that not only expresses magnitude but also motion or position of an object in relation to another point or object. Euclidean vector, geometric vector, and spatial vector are other names for it.

In mathematics, a vector's magnitude is defined as the length of a segment of a directed line, and the vector's direction is indicated by the angle at which the vector is inclined.

Know more about vector  here:

https://brainly.com/question/29740341

#SPJ11

encontre as raízes quadradas dos números:
a)²√625​

Answers

25

porque al multiplicar 25×25 nos da 625

Answer:

25

Step-by-step explanation:

25x25=625

CORRECT ANSWER GETS BRAINLIEST
Explain how you would apply exponent rules to evaluate for the value of (x³/x)² if x =1/2?

Answers

Answer:

1/16

Step-by-step explanation:

Picture is attached. The quickest way is to simplify BEFORE plugging in x = 1/2

Kai's mom gave him $25 to spend at the book fair. Kai wants to buy as many books as he can. The
cost of each book is $3. He also wants to buy one poster for $2. Write an inequality to show how
many books he can buy. Highlight your inequality in green. Then,solve the inequality

Answers

Answer:

7 books

Step-by-step explanation:

i cant write it in an equaction but basically 25 - 2 = 23

then 3 x 7 = 21 so he can only buy 7 books and he'll have 2 dollars left

3x + 2 = 25. x= 7

3x +2 =25
-2 -2
23
3x = 23


23 divided by 3 = 7

Find the basis for the kernel and imagine of this transformation. Let T : P₂ → R where T(p(x)) = p(1). Find a or the kernel and image of this transformation.

Answers

The basis for Ker(T) is {}, and the basis for Im(T) is {1}.

Since, The kernel of T, denoted Ker(T), is the set of all elements in P₂ that are mapped to zero in R by T.

In other words, Ker(T) is the set of all polynomials in P₂ such that

T(p(x)) = p(1) = 0.

Hence, To find the kernel, we need to solve the equation p(1) = 0.

Since, The only polynomial that satisfies this condition is p(x) = 0.

Therefore, Ker(T) = {0}.

Now, the image of T.

The image of T is the set of all elements in R that are mapped to by some element in P₂ under T.

In other words, Im(T) is the set of all possible values of p(1) for all polynomials p(x) in P₂.

Some example: - If p(x) = 1 + 2x + x², then p(1) = 1 + 2 + 1 = 4.

So, we can see that Im(T) is the set of all real numbers.

Hence, the basis for the kernel and image of T.

Since Ker(T) = {0}, the basis for the kernel is the empty set, denoted by {}.

Since Im(T) is the set of all real numbers, any non-zero real number can be expressed as a scalar multiple of any other non-zero real number.

Therefore, the basis for Ker(T) is {}, and the basis for Im(T) is {1}.

Learn more about the standard deviation visit:

https://brainly.com/question/475676

#SPJ4

A password is required to log into a system that only recognizes numbers and letters of the alphabet (not case sensitive). You want to create a password using 5 numbers or letters, but cannot reuse any of them. How many different passwords are possible?

Answers

There are 60,466,176 possible passwords for a system that acknowledges both digits and letters of the alphabet (and does not differentiate between uppercase and lowercase letters) and requires a 5-character password with no repeated characters.

For the purpose of computing the total number of possible passwords, it is necessary to take into account the number of options available for each character position. Within the constraints of the system, there are 10 different options for each number (0-9) and 26 different options for each letter (A-Z).

There are a total of 36 options available to us when it comes to the first character position (26 letters and 10 numerals). We are left with 35 options for the second place because we are unable to recycle any of the characters (36 options, minus the one that has already been used). In a similar fashion, we have 34 options for the third position, 33 options for the fourth position, and 32 options for the fifth position.

To calculate the total number of possible passwords, we multiply the number of choices for each position together: 36 * 35 * 34 * 33 * 32 = 60,466,176. Given the parameters of this system, there are consequently sixty-four million, four hundred sixty-six thousand, one hundred seventy-seven different passwords that might be used.

Learn more about number here:

https://brainly.com/question/3589540

#SPJ11

Can someone please help me with math

Answers

Answer:

A. 20.2 ft

Step-by-step explanation:

use the pythagorean theorem to get your answer.

a^2+b^2=c^2

17^2+11^2=c^2

289+121=410

***IMPORTANT, remember to square root the answer. 410 isn't your final answer, it's still in c^2 form. 20.2 is the correct answer after square rooting it. don't forget that for future problems

Answer:

A. 20.2 ft

Step-by-step explanation:

Hope it helps..

Caitlin ate 20 pieces of candy on Monday and 17 pieces on Tuesday. by what percent did she decrease her candy intake

Answers

She decreased her candy intake by 15%

find the value of x for both problems.​

Answers

Answer:

Step-by-step explanation:

In Problem 6, 3/x = tan 30 degrees, or x/3 = 1 / (tan 30 degrees), or

x =   3 / (tan 30 degrees).  Using a calculator:  x = 3 / (0.449) = 6.68 units

In Problem 9, we begin by finding the length of the hypotenuse of the upper triangle:  sin 60 degrees = 9√3 / hypotenuse, or

hypotenuse = 9√3/ (√3 / 2) = 9/2.  Then the hypotenuse of the 45-45-90 triangle is 9/2.  Thus, cos 45 degrees = x / (9/2), or

2x      √3

---- = -------

 9        2

                                                                                    9√3

Cross-multiplying, we get 4x = 9√3, so that x = -------------

                                                                                        4

Suppose that 5% of athletes at a certain university are using steroids or other
performance-enhancing drugs (PEDs). Unfortunately, the tests used to detect PEDs
aren't perfect. Suppose that 3% of athletes who are not using PEDs will test positive
and be accused of using PEDs. Meanwhile, 1% of athletes who are using PEDs will test
negative and escape detection. Calculate the probability that a randomly selected
athlete will test positive.

Answers

Answer:

There is a 8.11% of any randomly selected athlete to test positive.

Step-by-step explanation:

To make it easier, let's assume there are 100 athletes

5/100 have it

3/95 do not have it, but will test positive

1% of the 5 athletes who are using PEDs = 0.05%

This means that 4.95/100 will test positive

3/95 = 3.16%

(rounded, if you need a more precise answer, it's 3.1578947368421053)

3.16% + 4.95% = 8.11%

ABM Services paid a $4.15 annual dividend on a day it closed at a price of $54 per share. What
was the yield?

Answers

Answer:

the yield is 7.68%

Step-by-step explanation:

The calculation of the yield is given  below:

we know that

Yield is

= Annual dividend ÷ Price per share

= $4.15 ÷ $54

= 7.68%

Here we divide the annual dividend by the price per share in order to determine the yield  

hence, the yield is 7.68%  

And, the same would be considered

Use the box method to distribute and simplify (2 + 1)(2x - 3). Drag and drop the
terms to the correct locations of the table.

Answers

Answer:

2x^2-x-3

(2x)(-3)

(x)

(1)

Step-by-step explanation:

Distribute

(2x^2) (-3x)

(2x) (-3)

Methods to rid a model of multicollinearity include:

Answers

Methods to rid a model of multicollinearity include:  Include determination,Collection of data,Variable change,Regression ridge

Multicollinearity in a model can be addressed in a number of ways, including:

Include determination: Eliminating at least one connected factors can assist with mitigating multicollinearity. Stepwise regression or LASSO regression can be used to select the most important features by analyzing the correlation matrix.

Collection of data: By introducing variation in the predictors, more diverse and independent data can help reduce multicollinearity. Expanding test size can likewise help in relieving multicollinearity impacts.

Variable change: Multicollinearity can be reduced by transforming variables using centering, standardization, or power or logarithmic transformations. These changes change the scale or type of the factors, making them less associated.

Regression ridge: By adding a penalty term to the least squares estimation, ridge regression reduces the impact of multicollinearity and reduces the coefficients. This regularization method balances out the appraisals and works on model execution.

PCA, or Principal Component Analysis,: PCA can be used to generate uncorrelated linear combinations of the starting variables. These new factors, known as head parts, can be utilized as indicators in the model, successfully lessening multicollinearity.

To know more about Multicollinearity refer to

https://brainly.com/question/30691253

#SPJ11

1.02 1.05 1.08 1.00 1.00 1.06 1.08 1.01 1.04 1.07 1,00 kg Does this support the hypothesis, at 5%, that the mean weight for the whole batch is over 1.00 kg? 7.4 The expected lifetime of electric bulbs produced by a given process was 1500 hours. To test a new batch a sample of 10 was taken. This showed a mean lifetime of 1455 hours. The standard deviation of the production is known to still be 90 hours. Test the hypothesis, at 1% significance, that the mean lifetime of the electric light bulbs has not changed. 7.5 A sleeping drug and a neutral control were tested in turn on a random sample of 10 patients in a hospital. The data below represent the differences between the number of hours sleep under the drug and the neutral control for each patient: 2.0 0.2 -0.4 0.3 0.7 1.2 0.6 1.8 -0.2 1.0 hours Test, at 5%, the hypothesis that the drug would give more hours sleep on average than the control for all the patients in the hospital. 7.6 A coin is suspected of being biased. It is tossed 200 times and 114 heads occur. Carry out a hypothesis test to see whether the coin is indeed biased at 1% significance for anle of sight employees both?

Answers

1.02 1.05 1.08 1.00 1.00 1.06 1.08 1.01 1.04 1.07 1,00 kg: To test the hypothesis, at 5% significance, that the mean weight for the whole batch is over 1.00 kg, we can perform a one-sample t-test. The null hypothesis (H0) assumes that the mean weight is 1.00 kg, while the alternative hypothesis (H1) assumes that the mean weight is greater than 1.00 kg. By calculating the sample mean, sample standard deviation, and the test statistic, we can compare it with the critical value to determine if there is sufficient evidence to support the alternative hypothesis.

7.4: To test the hypothesis, at 1% significance, that the mean lifetime of electric light bulbs has not changed, we can use a one-sample t-test. The null hypothesis (H0) assumes that the mean lifetime is 1500 hours, while the alternative hypothesis (H1) assumes that the mean lifetime is different from 1500 hours. By calculating the sample mean, known standard deviation, and the test statistic, we can compare it with the critical value to determine if there is enough evidence to reject the null hypothesis.

7.5: To test the hypothesis, at 5% significance, that the drug gives more hours of sleep on average than the control for all patients in the hospital, we can perform a one-sample t-test. The null hypothesis (H0) assumes that there is no difference in the mean hours of sleep between the drug and the control, while the alternative hypothesis (H1) assumes that the drug provides more hours of sleep on average. By calculating the sample mean, sample standard deviation, and the test statistic, we can compare it with the critical value to determine if there is sufficient evidence to support the alternative hypothesis.

7.6: To test the hypothesis, at 1% significance, that the coin is biased, we can use a binomial test. The null hypothesis (H0) assumes that the coin is unbiased (probability of heads = 0.5), while the alternative hypothesis (H1) assumes that the coin is biased. By calculating the number of heads observed, the expected number of heads under the null hypothesis, and the test statistic, we can compare it with the critical value to determine if there is enough evidence to reject the null hypothesis and conclude that the coin is indeed biased.

To know more about the standard deviation, click here;

https://brainly.com/question/29115611

#SPJ11

A tank is full of water. Find the work W required to pump the water out of the spout. (Use 9.8 m/s2 for g. Use 1000 kg/m3 as the weight density of water.) 8 m Step 1 A layer of water Ax m thick which lies x m above the bottom of the tank will be rectangular with length 8 m Using similar triangles, we can see that it will have width 8r 8 m Step 2 The mass of the layer of water is approximately equal to its density (1000 kg/m3) times its approximate volume x Ax(1000) kg m=

Answers

The work required to pump the water out of the spout is 200000.64 J.

Given, Length of the tank = 8 m

Density of water = 1000 kg/m3

The work required to pump the water out of the spout can be calculated as follows:

Step 1: Consider a layer of water 'dx' thick at a height of 'x' meter above the bottom of the tank.

The volume of this layer is given by,V = Area × height= (8 × x) × dx= 8x dx

The mass of this layer is given by,m = density × volume= 1000 × 8x dx= 8000x dx

The force required to lift this layer of water is given by, F = mg= 8000x dx × 9.8= 78400x dx

Step 2: To find the work done, we need to multiply the force by the distance moved.

The distance moved by this layer of water is given by d, where d = (8 - x).

Therefore, the work done in moving this layer of water is given by, dW = F × d= 78400x dx × (8 - x)= 627200x dx - 78400x² dx

Step 3: The total work done in pumping out all the water is given by the integral of dW from x = 0 to x = 8.

That is,W = ∫dW = ∫₀⁸ (627200x dx - 78400x² dx)= [313600x² - 261333.33x³]₀⁸= 200000.64 J

Therefore, the work required to pump the water out of the spout is 200000.64 J.

know more about work done

https://brainly.com/question/13662169

#SPJ11

¿Cuáles son los números primos entre 2 y 20?

Answers

Answer:

2 3 5 7

Step-by-step explanation:

We know the following about the transformation A:
A ï1 0 ò=ï2 0 ò
and A ï0 1 ò=ï1 1 ò
73.1 Draw C2 and A(C2), the image of the unit square under A.
73.2 Compute the area of A(C2).
73.3 Compute det(A).

Answers

73.1 A(C2) is a quadrilateral with vertices (0, 0), (1, 0), (1, 2), and (0, 2).

73.2 The area of A(C2) is 2.

73.3 The determinant of A is 0

To draw C2 and A(C2),  to understand the transformation A and to the unit square.

Given the information:

A:

| 1 0 |

| 2 0 |

A maps the vector [1, 0] to [2, 0], and it maps the vector [0, 1] to [1, 1]. This provides us with the information to draw C2 and A(C2).

73.1 Draw C2 and A(C2):

C2 represents the unit square with vertices (0, 0), (1, 0), (1, 1), and (0, 1).

To draw A(C2), the transformation A to each vertex of C2 and connect the transformed vertices to form the image of C2 under A.

Applying A to the vertices of C2:

A(0, 0) = (1 × 0, 2 × 0) = (0, 0)

A(1, 0) = (1 × 1, 2 × 0) = (1, 0)

A(1, 1) = (1 × 1, 2 × 1) = (1, 2)

A(0, 1) = (1 × 0, 2 × 1) = (0, 2)

Connecting the transformed vertices, we get the image of C2 under A:

73.2 Compute the area of A(C2):

To compute the area of A(C2),  divide it into two triangles and calculate their areas separately.

Triangle 1: (0, 0), (1, 0), (1, 2)

Base of Triangle 1 = 1 - 0 = 1

Height of Triangle 1 = 2 - 0 = 2

Area of Triangle 1 = (1/2) × Base × Height = (1/2) × 1× 2 = 1

Triangle 2: (0, 0), (0, 2), (1, 2)

Base of Triangle 2 = 1 - 0 = 1

Height of Triangle 2 = 2 - 0 = 2

Area of Triangle 2 = (1/2) × Base × Height = (1/2) × 1 × 2 = 1

Total Area of A(C2) = Area of Triangle 1 + Area of Triangle 2 = 1 + 1 = 2

73.3 Compute det(A):

To compute the determinant of A (denoted as det(A)), use the formula for a 2x2 matrix.

A:

| 1 0 |

| 2 0 |

det(A) = (1 ×0) - (0 × 2) = 0 - 0 = 0

To know more about  area here

https://brainly.com/question/1631786

#SPJ4

Im giving brainliest

Answers

Answer:

√17 - 3   >   -2 + √5

find the area of this polygon

Answers

Answer:

answer is 65 in²

hope this helps

3/5 + 7/8 + 3/10
please help

Answers

Answer:

soln,

3/5+7/8+3/10

3×8/5×8+7×5/8×5+3×4/10×4

24/40+35/40+12/40

24+35+12/40

71/40

A rectangular prism has a base perimeter of 28 inches and a height of 6 inches. What is the lateral surface area of the prism?​

Answers

Answer:

uhuhuhuhuhuhuhuhuhuhuh oh yeah

Step-by-step explanation:

Can i pass 7th grade with 3 Fs?

Answers

Answer:

Depends on the state you’re from

Step-by-step explanation:

Answer:

Yes you can! Grades from middle school don't matter when you get to high school. But try to get those grades up!

Other Questions
(S)-Zearalenone, a natural product isolated from the fungus Gibberella zeae, exhibits useful biological activity, including antibiotic properties. In a total synthesis of (S)-zearalenone, an intramolecular Stille coupling process was employed as the penultimate procedure.3 The resulting coupling product was then treated with HCl to cleave the OR groups, thereby converting them into OH groups (as seen in Section 13.6), giving (S)-zearalenone. Draw the structure of (S)-zearalenone: QUESTION 24 Table 3-8 Assume that Pablo and Micah can switch between producing baseballs and producing bats at a constant rate. Labor Hours Needed Quantity Produced to Make 1. in 36 Hours baseball bat baseballs bats Pablo 1 3 36 12 Micah 1 2 36 18 Refer to Table 3-8. Micah has an absolute advantage in the production of O a bats and a comparative advantage in the production of baseballs. Ob neither good and a comparative advantage in the production of baseballs. O baseballs and a comparative advantage in the production of bats. Od bats and a comparative advantage in the production of bats. 1 point7.A tree has organs that carry out specific functions needed for survival.What function do the tree's leaves carry out? *A)act as reproductive structuresB)capture sunlight to make foodC)take in dissolved mineralsD)anchor the tree underneath the soil The distribution of actual weights of 8-ounce chocolate bars produced by a certain machine is Normal with mean 8.1 ounces and standard deviation 0.1 ounces. Company managers do not want the weight of a chocolate bar to fall below 8 ounces, for fear that consumers will complain. (a) Find the probability that the weight of a randomly selected candy bar is less than 8 ounces Forty candy bars are selected at random and their mean weight is computed. (b) Calculate the mean and standard deviation of the sampling distribution of (c) Find the probability that the mean weight of the forty candy bars is less than 8 ounces. (d) Would your answers to (a), (b), or (c) be affected if the weights of chocolate bars produced by this machine were distinctly non-Normal? Explain. GIVING BRAINLIEST!!!!!!The sky a blue blanket stretched beyond the horizen and beyond the pacific ocean.Which sentence is edited correctly? The sky, a blue blanket, stretched beyond the horizon and beyond the Pacific Ocean. The sky a blue blanket, stretched beyond the horizon, and beyond the pacific ocean. The sky, a blue blanket stretched beyond the horizon and beyond the Pacific ocean. The sky a blue blanket stretched, beyond the horizon, and beyond the pacific ocean. Given the line y = 3x + 5:Write the equation of a line, in point-slope form, that is parallel to the original and passes through the point (7,1) anyone know the answer How many liters of oxygen are there in 5 grams of oxygen at STP? A bag contains 12 yellow tiles and 12 blue tiles. A student will choose one tile from the bag without looking. Which word(s) describe the probability of choosing a blue tile from the bag? likely O certain O impossible O equally likelyon a k12 quiz btw In a paragraph, discuss 3 factors that could affect your decision to buy a certain appliance Which of these is an example of a vision statement?A. Educate our customers on our processes to help build customerloyaltyB. Strive for profits without compromising our values.C. To experience the emotion of competition, winning, and crushingcompetitors.D. We love making our jeans and we know people love wearing them. recipe calls for 1/3 of a cup of milk for 10 cookies how many cups of milk are needed to make 90 cookies Research by the Ethics Resource Center (ERC) found that 86 percent of the employees in companies with a well-implemented ethics and compliance program are likely to perceive a strong ethical culture within the company, while less than 25 percent of employees in companies with little to no program are likely to perceive a culture that promotes integrity in the workplace. Why are the ethics important in the corporate work place and how does it contribute to the business success . Find the amount necessary to fund the given withdrawals. Semiannual withdrawals of 270$ for 6 years, interest rate is 6.2% compounded semiannually Write 3 6 (three to the 6th power) as an equivalent expression using repeated multiplication. you have a coupon for $4 off the price of any large pizza . you also get a 20% discount on any pizza if you show your student ID. How much would you pay for a large pizza if the cashier applies the coupon first? 1. Briefly explain how the cloud type for any particular day can be determined.2. Name TWO types of rain clouds.giving away brainlist match the lines l1 (blue), l2 ( red) and l3 (green) with the slopes by placing the letter of the slopes next to each set listed below: The ratio of an object's weight on Earth to its weight on Venus is 8:7. How much would a woman who weighs 120 pounds (lbs) on Earth weigh on Venus? (Please help, if the answer is correct, I'll mark brainliest.) Of the 6 carbons in a starting molecule of glucose, at the completion of pyruvate oxidation, _____ carbons are fully oxidized to _____, while _____ carbons remain in _____. a. Three; pyruvate; three; acetyl groupsb. Two; CO2; four; acetyl groupsc. Four; CO2; two; CoAd. Two; acetyl groups; four; CO2e. Four; acetyl groups; two; CO2