The figure shows a smooth tunnel AB of length $$2\ell$$ in a uniform density planet of mass $$M$$ and radius $$R$$. A small ball of mass $$m$$ is released from rest at the end A of the tunnel. Acceleration due to gravity at surface of the planet is $$g$$. Time taken by the ball to reach the end B is

A

$$\pi\sqrt{\frac{R}{g}}$$

B

$$2\sqrt{\frac{l}{g}}$$

C

$$\frac{\pi}{2}\sqrt{\frac{2R}{g}}$$

D

$$2\pi\sqrt{\frac{R}{g}}$$