Concerning Inference on Two Independent Population Means, the Pooled Variance

Concerning inference on two independent population means, the pooled variance estimate is:

A) a way of estimating variance that relies on the information provided by only one of the samples.
B) an unbiased estimator of the parameter .
C) a combined (using a weighted average) estimate for the unknown variance of two populations.
D) a less efficient but easier way of estimating population variances as compared to using two separate variance estimates (one for each population) .