A Simulate thousand samples of size 1 (5, 10, 100, 1000) normally distributed variables with mean zero and variance of 4. Calculate the respective sample averages. Make a histogram of the sample averages. Comment on the graphs as you increase the size of the samples.

B The same as in the previous sub-exercise, but take the ratio of two standard-normally distributed variables. Comment on the graphs as you increase the size of the samples. When you focus on the samples of size 1, in which way does the graph vary compared to the first subexercise? In which circumstances could a distribution like this one apply?

C The same as in the previous sub-exercise but with following variation: keep the standardnormally distributed variable in the numerator. In the denominator, add up two squared standard normally distributed variables, divide by two, take the square root of that. Comment on the graphs as you increase the size of the samples.  


D Repeat the above three sub-exercises, but calculate and graph the sample variance instead of the sample mean.  

问题如标题，网上说sum之后就直接histogram就成，所以mean和variance的图一样？不是的话，要用什么命令？