An object of mass m starts falling towards planet of mass M and Radius R. As it reaches near the surface it passes through a small hole in the planet. The planet is made of two pieces, a sperical shell of negligible thickness and mass $\large\frac{2M}{3}$ and a point mass of $\large\frac{M}{3}$ at center. Change in the force of gravity experienced by the object is

Question

$$(a)\;\frac{2}{3}\frac{GMm}{R^2} \quad (b)\;0 \quad (c)\;\frac{1}{3}\frac{GM^2}{R^2} \quad (d)\;\frac{4}{3}\frac{GMm}{R^2}$$