真的很晕，不知道如何下手，请高人指教. 多谢！

110.      11364-!-item-!-187;#058&007646

The residents of Town X participated in a survey to determine the number of hours per week each resident spent watching television.  The distribution of the results of the survey had a mean of 21 hours and a standard deviation 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 television last week?

(A) 30

(B) 20

(C) 18

(D) 12

(E) 6

答案D

这个题的重点在the # of hours per week Pat watched TV was between 1 and 2

standard deviations below the mean.

one standard deviation below the mean = 21-6=15
two standard deviations below the mean = 21-6*2=9

所以Pat看电视的时间在9小时和15小时之间，只有答案D符合。