Three identical metallic uncharged spheres $A,B$ and $C$ of radius a are kept on the corners of an equilateral triangle of side $d(d>>a)$. A fourth sphere (radius $a$ ) which has charge $Q$ touches $A$ and is then removed to a position far away. $B$ is earthed and then the earth connection is removed. $C$ is then earthed. The charge on $C$ is