Two masses $M_1$ and $M_2$ are arranged as shown in the figure. Let '$a$' be the magnitude of the acceleration of the system. If the mass of $M_1$ is doubled and that of $M_2$ is halved,then the new acceleration of the system is (Treat all surfaces as smooth; masses of pulley and rope are negligible).

• A
  $\left(\frac{M_1+M_2}{4 M_1+M_2}\right) a$
• B
  $\left(\frac{2 M_1+M_2}{4 M_1+M_2}\right) a$
• C
  $\left(\frac{M_1+2 M_2}{4 M_1+2 M_2}\right) a$
• D
  $\left(\frac{M_1+2 M_2}{M_1+M_2}\right) a$