In the Equations for a T-Test in Matched or Dependent $t_{a k t}=\frac{\bar{X}_{D}}{s_{D} / \sqrt{n}}$

In the equations for a t-test in matched or dependent samples, $t_{a k t}=\frac{\bar{X}_{D}}{s_{D} / \sqrt{n}}$ , $\bar{X}_{D}$ is ______.

A) the standard deviation
B) the variance
C) the mean of one group
D) the mean of the difference between each pair of scores in the sample