Question

The two particles are both moving to the right. Particle 1 catches up with particle 2 and collides with it. The particles stick together and continue on with velocity vf. Which of these statements is true?
A. vf=v2. B. vf is less than v2. C. vf is greater than v2, but less than v1. D. vf=v1. E. vf is greater than v1.