A smooth tunnel is dug along the radius of earth that ends at center. A ball is released from the surface of earth along tunnel. What is the velocity when it strike the center?

Question

\[(a)\;v=\sqrt {\frac{GR}{2M}} \quad (b)\;v=\sqrt {\frac{GM}{2R}} \quad (c)\;v=\sqrt {\frac{2GM}{R}} \quad (d)\;v=\sqrt {\frac{GM}{R}} \]