Two blocks of masses 4 kg and 3 kg are placed on a friction less surface connected by a string. An external force gives a velocity 7 m/s to the heavier mass in the direction of the lighter , velocity of their centre of mass is

Question

$$\begin {array} {1 1} (a)\;7m/s & \quad (b)\;3.5m/s \\ (c)\;3m/s & \quad  (d)\;4m/s \end {array}$$