3 particle each of mass $\mathrm{m}$ are kept at vertices of an equilateral triangle of side $L$. The gravitational field at center due to these particles is
(A) zero
(B) $\left[(3 \mathrm{GM}) / \mathrm{L}^{2}\right]$
(C) $\left[(9 \mathrm{GM}) / \mathrm{L}^{2}\right]$
(D) $(12 / \sqrt{3})\left(\mathrm{Gm} / \mathrm{L}^{2}\right)$