If Two Random Samples of Sizes 30 and 36 Are $\bar { X } _ { 1 } - \bar { X } _ { 2 }$

If two random samples of sizes 30 and 36 are selected independently from two populations with means 78 and 85, and standard deviations 12 and 15, respectively, then the mean of the difference $\bar { X } _ { 1 } - \bar { X } _ { 2 }$ is equal to:

A) -7
B) 7
C) (78 - 85) / (30 -36) = 1.17
D) 78/30 - 85/36 = 0.24