请牛牛们帮我解答，prep1-106数学

106  If a certain sample fo data has a mean of 20.0and a standard deviation of 3.0, which of the following values is more 2.5 standard deviations from the mean?

A 12.0

B 13.5

C 17.0

D 23.5

E 26.5

答案是A

请牛牛们帮我解答，谢谢了！

If a certain sample
of data has a mean of 20.0 and a standard deviation of 3.0, which of the
following values is more than
        2.5
standard deviations from the mean?

(B) 13.5

(C) 17.0

(D) 23.5

(E) 26.5