11. The mass density of a certain planet has spherical symmetry but varies in such a way that the mass inside every spherical surface with center at the center of the planet is proportional to the radius of the surface. If r is the distance from the center of the planet to a point mass inside the planet, the gravitational force on the mass is(A)not dependent on r(B)proportional to \( r^2 \)(C)proportional to r(D)proportional to \( \frac{1}{r} \)(E)proportional to \( \frac{1}{r^2} \)