Which of the Following Best Describes the Symbol $\bar { x } _ { \mathrm { D } }$

Which of the following best describes the symbol $\bar { x } _ { \mathrm { D } }$ ? $\begin{array}{|l|l|}\hline \text { A. } & \text { The difference in the means of two dependent populations. } \\\hline \text { B. } & \text { The difference in the means of two independent populations. } \\\hline \text { C. } & \begin{array}{l}\text { The mean of the differences in the pairs of observations taken from two dependent } \\\text { samples. }\end{array} \\\hline \text { D. } & \text { The matched pairs differences. } \\\hline\end{array}$