A simple pendulum is suspended from the roof of a trolley which moves in a horizontal direction with an acceleration a, then the time period is given by $\mathrm{T}=2\mathrm{\pi }\sqrt{\frac{\mathrm{l}}{{\mathrm{g}}^{\text{'}}}}$,  where  ${\mathrm{g}}^{\text{'}}$ is equal to 

• A

  g

• B

  $\mathrm{g}-\mathrm{a}$

• C

  $\mathrm{g}+\mathrm{a}$

• D

  $\sqrt{{\mathrm{g}}^2+{\mathrm{a}}^2}$