Pooling is used in calculating the standard error for the difference between means when the population standard deviations are unknown, for which of the following reasons? (MORE THAN ONE OPTION IS CORRECT)

A. because the weighted average of the two sample variances gives a better estimate of the common population variance

B. because the weighted average of the two population variances gives a better estimate of the common sample variance

C. because the variances in the populations are assumed equal

D. because the null hypothesis usually shows there is no difference between the means