6. 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 particle of mass m located a distance d from the center, outside the inner radius and inside the outer radius, is: (A)0 (B)GMm/d² (C)GMm(R₂²-R₁³) (D)GMm(d³-R₁³)/[d²(R₂³-R₁³)] (E)GMm/(d²-R₁³)