还有一题prep 晕？

the residents of town x participated in a survey to determine the number of hours per week each resident spend watching tv,the distribution of the results of the survey had a mean of 21 hours and a stand deviation of 6 hours.the number of hours the pat, a resident of town x, watched tv 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 tv last week?

30 , 20, 18, 12,  6

是12吗？一倍Std.范围是15(21-6)，两倍是9，在1，2倍之间，应该是在9到15之间吧。

是12