You are programming a game that perfectly features elastic collision. A red ball is traveling
along the x-axis with a speed of +5 and a blue ball is traveling along the x-axis with a speed
of -5. There is no motion on any other axis. Each ball has identical mass. They collide with
the center of each ball perfectly aligned with the other.
What reaction motion should you expect after the collision?

A.
Both balls begin moving along the y-axis at a speed of -S.

B.
Both balls stop, and there is no more motion.

C.
The red ball stops, and the blue ball begins moving at a speed of +10 along the x-axis.

D.
The red ball begins moving along the x-axis at a speed of -5, and the blue ball begins
moving along the x-axis at a speed of + 5.

Explanation:
http://stackoverflow.com/questions/345838/ball-to-ball-collision-detection-and-handling