A particle of mass M is situated at the center of a spherical shell of same mass and radius a. The gravitational potential at a point situated at $\frac{a}{2}$ distance from the centre, will be –

Question

( A ) $-\frac{3GM}{a}$
( B ) $-\frac{GM}{a}$
( C ) $-\frac{4GM}{a}$
( D ) $-\frac{2GM}{a}$