Given a text T, and several options, according to the question posed in the text T to choose the most appropriate option from the options as the answer, please directly output the most appropriate option, do not need to output the explanation.

A pooled estimator is used when testing a hypothesis concerning the:
Option A:equality of the variances of two normally distributed populations.
Option B:difference between the means of two at least approximately normally distributed populations with unknown but assumed equal variances.
Option C:difference between the means of two at least approximately normally distributed populations with unknown and assumed unequal variances.

Answer: B