Two Identical Freight Cars Roll Without Friction Towards Each Other

Two identical freight cars roll without friction towards each other on a level track.One car rolls at 2 m/s and the other car rolls at 1 m/s.After the cars collide,they couple and roll together with a speed of

A) 0.5 m/s.
B) 0.33 m/s.
C) 0.67 m/s.
D) 1.0 m/s.
E) none of the above