Two Identical Gliders Slide Toward Each Other on an Air

Multiple Choice

Two identical gliders slide toward each other on an air track. One glider moves at 1 m/s and the other glider moves at 2 m/s. They collide and stick. The combined gliders move at

A) 1/2 m/s.
B) 1/3 m/s.
C) 1/6 m/s.
D) 3/4 m/s.
E) 1.5 m/s.