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

In testing the difference between two population means, using two independent samples, the sampling distribution of the sample mean difference $\bar { x } _ { 1 } - \bar { x } _ { 2 }$ is normal if the: $\begin{array}{|l|l|}\hline A.&\text {sample sizes are both greater than 30 . }\\\hline B.&\text { popul ations are normal.}\\\hline C.&\text {populations are non-normal and the sample sizes are large. }\\\hline D.&\text {All of these choices are correct. }\\\hline \end{array}$