The acceleration due to gravity g and mean density of the earth ρ are related by which of the following relations? (where G is the gravitational constant and -R is the radius of the earth.)
(a) ρ = 3g/4π GR
(b) ρ = 3g/4π GR3
(c) ρ = 4π GR2/3G
(d) ρ = 4π gR3/3G
