[求助]PREP上的一道题，不明白

kikijesse

题目是这样的：

The residents of Town X participated in a survey to determine the number of hours per week each resident spent watch television. The distribution of the results of the survey had a mean of 21 hours and a standard diviation of 6 hours. The number of hours that Pat, a resident of Town X, watched television last week was between 1 and 2 standard deviations below the mean. Which of the following could be the number of hours that Pat watched last week? 

A. 30

B. 20

C. 18

D. 12

E. 6

答案选D
        

首先我对红色部分的题目不是很理解，想请教各位牛牛~~另外，这个怎么算出来的呢？谢谢了~~

leopoldfang

意思好像是在这个人看电视的时间落在均值下的1-2个方差区间中，即区间（21-2*6，21-6）=（9，15），所以选D哈

kikijesse

哦~~是不是就是考那个，方差是各个数字离均值的游离程度啊~~