A Railway Car Having a Total Mass of 5 $\times$ 10⁵ Kg,moving with a Speed of 9

Multiple Choice

A railway car having a total mass of 5.8 $\times$ 10⁵ kg,moving with a speed of 9.1 km/h,strikes another car that has a mass of 8.7 $\times$ 10⁵ kg and is initially at rest.The speed of the coupled cars after the collision is

A) 9.1 km/h
B) 7.2 km/h
C) 3.6 km/h
D) 1.8 km/h
E) 4.2 km/h