$A$ simple pendulum is set up in a trolley which moves to the right with an acceleration $a$ on a horizontal plane. Then the thread of the pendulum in the mean position makes an angle $\theta$ with the vertical.

• A
  $\tan^{-1}(\frac{a}{g})$ in the forward direction
• B
  $\tan^{-1}(\frac{a}{g})$ in the backward direction
• C
  $\tan^{-1}(\frac{g}{a})$ in the backward direction
• D
  $\tan^{-1}(\frac{g}{a})$ in the forward direction