Independent samples are obtained from two normal populations with unknown but equal variances in order to construct a hypothesis test for the difference between the population means. If the first sample contains 16 items and the second sample contains 26 items, the correct form to use for the sampling distribution is the

A. normal distribution

B. t distribution with 15 degrees of freedom

C. t distribution with 25 degrees of freedom

D. t distribution with 40 degrees of freedom

E. none of the above