In Constructing a Confidence Interval Estimate for the Difference Between

In constructing a confidence interval estimate for the difference between the means of two normally distributed populations, using two independent samples, we: $\begin{array}{|l|l|}\hline A&\text { pool the sample variances when the unknown population variances are equal.}\\\hline B&\text {pool the sample variances when the populati on variances are known and equal. }\\\hline C&\text { pool the sample variances when the population means are equal. }\\\hline D&\text { never pool the sample variances.}\\\hline \end{array}$