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 rolls at 2 m/s and the other 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