Two metal spheres of same material and each of radius r are in contact with each other. The gravitational force of attraction between the sphere is proportional to

Explanation

`F=(Gm_1m_2)/((2r)^2)=(G({:(4),(3):}pir^3d)({:(4),(3):}pir^3d))/(4r^2)`
`F = Kr^4" " F propr^4`