Two 5000-kg freight cars roll toward each other (one at 1 m/s, the other at 2 m/s) on a level track. Thus before they collide one car's kinetic energy is 2500 J and the other's is 10,000 J. After they collide (and couple together) their total kinetic energy is ?

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