Two 5000-Kg Rail Cars Roll Without Friction (One at 1

Two 5000-kg rail cars roll without friction (one at 1 m/s, the other at 2 m/s) toward each other on a level track. They collide, couple, and roll together with a combined momentum of

A) zero.
B) 5000 kg m/s.
C) 10,000 kg m/s.
D) 15,000 kg m/s.