The mass density of a certain planet has spherical symmetry but varies in such a way that mass inside every spherical surface with the centre at the centre of planet is proportional to the radius of surface. If 'r' is the distance from the centre of planet to a point mass inside the planet, the gravitational force on the mass is proportional to -
1
r2
2
r
3
\(\frac{1}{r}\)
4
\(\frac{1}{r^2}\)