If Two Random Samples of Sizes N₁ and N₂ Are $\mu$

If two random samples of sizes n₁ and n₂ are selected independently from two populations with means $\mu$ ₁ and $\mu$ ₂, then the mean of $\bar { X } _ { 1 } - \bar { X } _ { 2 }$ equals:

A) $\mu$ ₁ + $\mu$ ₂
B) $\mu$ ₁ - $\mu$ ₂
C) $\mu$ ₁ / $\mu$ ₂
D) $\mu$ ₁ $\mu$ ₂