还是分不清两个样本标准差-请教下

问题如下：

If the mean P/E of 30 stocks in a certain industrial sector is 18 and the sample standard deviation is 3.5, standard error of the mean is CLOSEST to:

选项：

A.

0.12

B.

0.34

C.

0.64

D.

1.56

解释：

C is correct.

考点 ： The Central Limit Theorem

解析 ： The standard error of the sample mean is the standard deviation of the distribution of the sample means. And it is calculated as:

So, standard error of the mean = $s/\sqrt n$ = $3.5/\sqrt{30}$ = 0.64