A spherical shell has inner radius R₁, outer radius R₂, and mass m, distributed uniformly throughout the shell. The magnitude of the gravitational force exerted on the shell by a point mass particle of mass m, located at a distance d from the center, inside the inner radius is :
a. 0
b. GM m/d²
c. GM m/(R₂³ - d³)
d. GM m (d³ - R₁³)/d²(R₂³ - R₁³)
e. GM m/(d³ - R₁³)