Two 5000-kg freight cars roll without friction (one at 1 m/s, the other at 2 m/s) toward one another on a level track. They collide, couple, and roll away 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.