A 2 g piece of gum is flicked straight at 5 m/s toward a 5 g block at rest on a frictionless horizontal surface. The collision is shown. The gum sticks to the block, and both move to the right. What is their speed?
a) 10 m/s
b) 2 m/s
c) 7.5 m/s
d) 5 m/s