Q2:D A certain characteristic in a large population has a distribution that is symmetric 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% 这题答案是D，我花了15分钟都没做出来，哪位好心给讲讲。。。 | 
