Two particles A and B initially at rest move towards each other under a mutual force of attraction. At the instant when the velocity of A is V and that of B is 2 V , the velocity of centre of mass of the system is :

Question

$\begin {array} {1 1} (1)\;Zero & \quad (2)\;V \\ (3)\;2V & \quad (4)\;3V \end {array}$