Three charges, each of value $Q$, are placed at the vertex of an equilateral triangle. A fourth charge $q$ is placed at the centre of the triangle. If the charges remains stationery then, $q=\ldots \ldots \ldots$
(A) $(\mathrm{Q} / \sqrt{2})$
(B) $-(\mathrm{Q} / \sqrt{3})$
(C) $-(Q / \sqrt{2})$
(D) $(\mathrm{Q} / \sqrt{3})$