A blue train of mass 50 kg moves at 4 m/s toward a green train of 30 kg initially at rest. The trains collide. After the collision the green train moves with a speed of 3 m/s. What is the final momentum of the blue train?
A. 110 kgm/s
B. 200 kgm/s
C. 20 kgm/s
D. 90 kgm/s