[求助]请教一道数学题

请教各位一道数学题,关于标准偏差的,这个好像中国的中学里都不怎么教,所以不太懂.

A certain characteristic in a large population has a distribution that is symmetric about the mean m. If 68% 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%