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 at least 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.
C选项为什么不对， C选项说的也是要用t检验的情况
