Populations x, y, and z are each normally distributed. their distributions are given in the graph below. which of the following statements comparing the means and standard deviations of the three populations is correct? a. population z has the largest standard deviation and population x has the smallest mean. b. populations x and y have the same mean. populations y and z have the same standard deviation. c. populations x and y have the same standard deviation, which is larger than that of z. d. populations y and z have the same mean, but population z has a standard deviation that is bigger than the standard deviation of y.