If Two Random Samples of Sizes $n _ { 1 }$

Multiple Choice

If two random samples of sizes $n _ { 1 }$ and $n _ { 2 }$ are selected independently from two non-normally distributed populations, then the sampling distribution of the sample mean difference, $\bar { X } _ { 1 } - \bar { X } _ { 2 }$ , is:

A) always non-normal.
B) always normal.
C) approximately normal only if $n _ { 1 }$ and $n _ { 2 }$ are both larger than 30.
D) approximately normal regardless of $n _ { 1 }$ and $n _ { 2 }$ .