SAT Data - Tables

$\begin{array}{|c|c|c|c|c|} \hline & \text{High School} & \text{Middle School} & \text{ Total}\\ \hline \text{Boys} & a & 2b & X\\ \hline \text{Girls} & 2a & b & Y\\ \hline \text{Total} & M & N & T\\ \hline \end{array}$

In the table above, each letter represents the number of students in that category. Which of the following equals $M-N?$

(A) $\ \ X$

(B) $\ \ Y$

(C) $\ \ 3(Y-X)$

(D) $\ \ \frac{T}{3}$

(E) $\ \ X+Y$