If R is the radius of the earth and g the acceleration due to gravity on the earth's surface, the mean density of the earth is

• A

  $\frac{4\mathrm{\pi G}}{3\mathrm{gR}}$

• B

  $\frac{3\mathrm{\pi R}}{4\mathrm{gG}}$

• C

  $\frac{3\mathrm{g}}{4\mathrm{\pi RG}}$

• D

  $\frac{\mathrm{\pi R}}{12\mathrm{G}}$