问一道，没碰到过的陌生题型，谢谢！

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

答案为什么是B? 谢谢！

肯定是题意没看明白吧~~ 题目意思是，已知10个学生的跑步成绩，并且给你算出了标准差。问10个成绩中秒数低于平均成绩一个标准差以上的有几个。

所以很简单，算出平均成绩100，减22.4，看有几个数比这个小。

Prep里面有这样一题，不能算陌生。

明白了，多谢！