When Figuring the Pooled Population Variance Estimate in a T

When figuring the pooled population variance estimate in a t test for independent means,

A) the variance of at least one of the original populations must be known,as opposed to estimated,but the other can be estimated from sample scores.
B) the variance of BOTH of the original populations must be known,as opposed to estimated.
C) the estimates based on each of the samples are averaged in such a way as to give more influence to the estimate based on more participants.
D) the scores from both samples are combined to form a single sample,and the estimated variance is figured in the usual way using this combined sample.