4. Two planets have the same radius, R. Each planet has a constant density, but the density of planet 2 is twice as high as that of planet 1. Identical objects of mass m are placed on the surfaces of the planets. What is the relationship of the gravitational potential energy U₁, on planet 1 to U₂ on planet 2? (A) U₁ = U₂ (B) U₁ = 2U₂ (C) U₁ = ½U₂ (D) U₁ = 4U₂.