Four particles of masses $m, 2 m, 3 m$ and $4 m$ are kept in sequence at the corners of a square of side $a$. The magnitude of gravitational force acting on a particle of mass $m$ placed at the centre of the square will be
(a) $\frac{24 m^{2} G}{a^{2}}$
(b) $\frac{6 m^{2} G}{a^{2}}$
(c) $\frac{4 \sqrt{2} G m^{2}}{a^{2}}$
(d) Zero