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:

A) normally distributed only if both population sizes are greater than 30.
B) normally distributed.
C) normally distributed only if at least one of the sample sizes is greater than 30.
D) approximately normally distributed.