A Sandbag Is Motionless in Outer Space

A sandbag is motionless in outer space.A second sandbag with 3 times the mass moving at 12 m/s collides with it.They stick together and move at a speed of

A) 3 m/s.
B) 4 m/s.
C) 6 m/s.
D) 8 m/s.
E) none of these