In the figure,two blocks $M$ and $m$ are tied together with an inextensible and light string. The mass $M$ is placed on a rough horizontal surface with coefficient of friction $\mu$ and the mass $m$ is hanging vertically against a smooth vertical wall. The pulley is frictionless. When the downward acceleration of the elevator becomes equal to $g$,then

• A
  both the blocks remain stationary with respect to the elevator
• B
  both the blocks accelerate vertically downwards with $g$ with respect to ground
• C
  the tension in the string becomes equal to zero
• D
  All of the above