If the value of ' $\mathrm{g}$ ' acceleration due to gravity, at earth surface is $10 \mathrm{~ms}^{-2}$. its value in $\mathrm{ms}^{-2}$ at the center of earth, which is assumed to be a sphere of Radius ' $\mathrm{R}$ 'meter and uniform density is
(A) 5
(B) $10 / \mathrm{R}$
(C) $10 / 2 \mathrm{R}$
(D) zero