In Testing the Difference Between Two Population Means Using Two $\overline { \mathrm { x } } _ { 1 } - \overline { \mathrm { x } } _ { 2 }$

Multiple Choice

In testing the difference between two population means using two independent samples,the sampling distribution of the sample mean difference $\overline { \mathrm { x } } _ { 1 } - \overline { \mathrm { x } } _ { 2 }$ is normal if the:

A) sample sizes are both greater than 30.
B) populations are normal.
C) populations are non-normal and the sample sizes are large.
D) population sizes are both greater than 30.
E) all of these are required conditions.