GMAT prep 数学求助,唉

               70,75,80,85,90,105, 105,130,130,130

The list shown consists of the times, in seconds, that it took each of 10 schoolchildren to run a distance of 400 meters. If the standard deviation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?

A.        one

B.         two

C.        three

D.        four

E.         five