请教一道prep里的数学问题，谢谢了

the list shown consists of times in seconds, that it took each of 10 girls to run 400m, it the standard deviation of the 10 running times is 22.4 seconds, rounded to 10th of a second, how many of the 10 running times are more than 1 standard deviation below the mean of 10 running times.

A, 1

B, 2

C, 3

D, 4,

E, 5

我怎么都看不懂问题how many of the 10 running times are more than 1 standard deviation below the mean of 10 running times.是什么意思，谢谢了