A freight car of mass 120,000 kg rolling down the track at 3 m / s collides with an identical in mass freight car that was initially at rest. The two cars couple together and move off together. Calculate the speed of the combination of two cars; treat it as a perfect inelastic collision. A. 3 m/s B. 1.5 m/s C. 4.5 m/s D. None of the above.