The Standard Deviation Of $\bar { X } _ { 1 } - \bar { X } _ { 2 }$

The standard deviation of $\bar { X } _ { 1 } - \bar { X } _ { 2 }$ is also called the:

A) standard error of the difference between two sample means.
B) standard deviation of the difference between the population means.
C) normal approximation to the difference of two binomial random variables.
D) None of these choices.