Two Identical Objects in Outer Space Have a Head-On Collision

Multiple Choice

Two identical objects in outer space have a head-on collision and stick together. If, before the collision, one had been moving at 2 m/s and the other at 1 m/s, their combined speed after the collision would be

A) 0.33 m/s.
B) 1.0 m/s.
C) 0.67 m/s.
D) 0.5 m/s.
E) none of these