In a two-sample t-test with equal variances, what is the pooled standard error for the difference of means?

In a two-sample t-test with equal variances, what is the pooled standard error for the difference of means?

• A. SE = sqrt( s1^2/n1 + s2^2/n2 )
• B. SE = sqrt( s_p^2 (1/n1 + 1/n2) )
• C. SE = sqrt( (s_p^2) / n1 + (s_p^2) / n2 )
• D. SE = sqrt( s_p^2 (1/n1 + 1/n2) ) where s_p^2 = [(n1−1)s1^2 + (n2−1)s2^2] / (n1+n2−2).

Answer: (D)