A plumb line is hanging from the ceiling of a train. If the train moving along a horizontal track with a uniform acceleration a, the plumb line gets inclined to the vertical at an angle $\theta$ is given by 

• A

  ${\tan }^{-1}\left(\frac{\mathrm{a}}{\mathrm{g}}\right)$

• B

  ${\tan }^{-1}\left(\frac{\mathrm{g}}{\mathrm{a}}\right)$

• C

  ${\sin }^{-1}\left(\frac{\mathrm{a}}{\mathrm{g}}\right)$

• D

  ${\cos }^{-1}\left(\frac{\mathrm{g}}{\mathrm{a}}\right)$