If Two Populations Are Not Known to Be Normally Distributed $\bar { X } _ { 1 } - \bar { X } _ { 2 }$

If two populations are not known to be normally distributed, the sampling distribution of the sample mean difference, $\bar { X } _ { 1 } - \bar { X } _ { 2 }$ , will be: $\begin{array}{|l|l|}\hline A&\text {normally distributed only if both population sizes are greater than 30 . }\\\hline B&\text {normally distributed. }\\\hline C&\text {normally distributed only if at least one of the sample sizes is greater than 30 . }\\\hline D&\text { approximately normally distributed.}\\\hline \end{array}$