请解释一下这道题

问题如下：

A population has a known mean of 1,000. Suppose 1,600 samples are randomly drawn (with replacement) from this population. The mean of the observed samples is 998.7, and the standard deviation of the observed samples is 100. What is the standard error of the sample mean?

选项：

A.

0.025

B.

0.25

C.

2.5

D.

25

解释：

This is $\frac\sigma{\sqrt t}$, or$\frac{100}{\sqrt{1600}}$ =$\frac{100}{40}$ = 2.5. Other numbers are irrelevant.