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 $\mathrm{\theta }$ with the vertical

• A

  ${\tan }^{-1}\frac{\mathrm{a}}{\mathrm{g}}$ in the forward direction

• B

  ${\tan }^{-1}\frac{\mathrm{a}}{\mathrm{g}}$ in the backward direction

• C

  ${\tan }^{-1}\frac{\mathrm{g}}{\mathrm{a}}$ in the backward direction

• D

  ${\tan }^{-1}\frac{\mathrm{g}}{\mathrm{a}}$ in the forward direction