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

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

A) $\mu _ { 1 }$ + $\mu _ { 2 }$ .
B) $\mu _ { 1 }$ - $\mu _ { 2 }$ .
C) $\mu _ { 1 }$ / $\mu _ { 2 }$ .
D) $\mu _ { 1 }$ $\mu _ { 2 }$ .: