A 5000-Kg Freight Car Crashes into a 10,000-Kg Freight Car

A 5000-kg freight car crashes into a 10,000-kg freight car at rest.They couple upon collision and move with a speed of 2 m/s.What was the initial speed of the 5000-kg freight car?

A) 4 m/s
B) 5 m/s
C) 6 m/s
D) 8 m/s
E) none of the above