Two particles A and B initially at rest, move towards each other, under mutual force of attraction. At an instance when the speed of A is 'v' and speed of B is '2v', the speed of centre of mass (c.m) is

Question

$$\begin {array} {1 1} (1)\;zero & \quad (2)\;v \\ (3)\;2.5 \;v & \quad (4)\;4\;v \end {array}$$