求助一道数学题！！！！

Gracespa

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%

答案是D

压根没懂什么意思。。。

Gracespa

谢啦~我以为自己哪里没想周全~

wanghonglia

是D

honoluluhono

两头各占（1-0.68）/2=0.16,（m-sigma,m+sigma）区域占0.68. 所以（-无穷，m+sigma）占的为0.16+0.68=0.84