$A$ mass $M$ is placed on a smooth wedge of angle $\theta$ resting on a frictionless surface. To keep the mass $M$ at rest relative to the wedge,the wedge must be given a horizontal acceleration $a$ equal to:

• A
  $a$ is applied to the right and $a=g \tan \theta$
• B
  $a$ is applied to the left and $a=g \sin \theta$
• C
  $a$ is applied to the left and $a=g \cos \theta$
• D
  $a$ is applied to the left and $a=g \tan \theta$