A particle of mass $\mathrm{M}$ is situated at the center of a spherical shell of same mass and radius a the magnitude of gravitational potential at a point situated at (a/2) distance from the center will be
(A) $[(4 \mathrm{GM}) / \mathrm{a}]$
(B) $(\mathrm{GM} / \mathrm{a})$
(C) $[(2 \mathrm{GM}) / \mathrm{a}]$
(D) $[(3 \mathrm{GM}) / \mathrm{a}]$