[求助]请教prep数学题standard deviation

原题：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 deviation below the mean. which of the following could be the number of hours that Pat watched television last week?

正确答案是12

希望牛人给出解题步骤，先谢过了^_^

这道题楼主应该是不理解 between 1 and 2 standard deviation below the mean，就是比平均数小一到二个标准差这个范围内。

设Pat上周看电视为X小时

21-2*6< X < 21-1*6 —> 9<x<15 然后看哪个选项在这个范围内，故选12.

谢谢