求高手解答GWD15套Math的第二题~~~！！！

mu1huizi

Q2:

A certain characteristic in a large population has a distribution that is symmetric(对称的) about the mean m.  If 68 percent of the distribution lies within one standard deviation d of the mean, what percent of the distribution is less than m +
d ?




A.   16%

B.    32%

C.   48%

D.   84%

E.    92%

Answer: D

做的时候连题目都没看懂，望大牛解答！感激不尽！！！