An Empty Railroad Car Traveling at a Speed of 21

Multiple Choice

An empty railroad car traveling at a speed of 21 m/s strikes a second car at rest.The second car,being full with passengers,has twice the mass of the first car.If the cars lock together as a result of the collision,their common speed afterward is

A) 11 m/s.
B) 10 m/s.
C) 9.0 m/s.
D) 8.0 m/s.
E) 7.0 m/s.