Two 5000-kg freight cars roll without friction (one at 1 m/s, the other at 2 m/s) toward each other on a level track. Thus one car's kinetic energy is 2500 J and the othe's is 10,000 J. Before they collide their total kinetic energy is ?

• A. 1250 J
• B. 2500 J
• C. 7500 J.
• D. 12,500 J.