A driver approaches a stoplight at 45 mph and is not paying attention. They fail to brake when they suddenly rear-end an identical car that was stopped at the light. The crash is so severe that the cars remain stuck together afterward. At what speed does the mangled mass of cars move just after the collision? A. vf = 90 mph B. vf = 45 mph C. vf = 22.5 mph D. vf = 0 mph E. Speed cannot be determined with the information given