请问可以辨析一下B和C吗？

问题如下：

A pooled estimator is used when testing a hypothesis concerning the:

选项：

A.

equality of the variances of two normally distributed populations.

B.

difference between the means of two approximately normally distributed

populations with unknown but assumed equal variances.

C.

difference between the means of two at least approximately normally distributed

populations with unknown and assumed unequal variances.

解释：

B is correct. The assumption that the variances are equal allows for the combining

of both samples to obtain a pooled estimate of the common variance.