In billiards, if no torque is supplied, when the moving cue ball strikes a stationary ball the two balls often move off at right angles after the collision. The best explanation for this is that

A) the collision is elastic and kinetic energy must be conserved.
B) the collision is inelastic and kinetic energy must be conserved.
C) the collision is elastic and potential energy must be conserved.
D) the collision is inelastic and potential energy must be conserved.