People who solve puzzles just for fun often reduce the amount of free-time they spend doing the puzzles if they are paid for puzzle-solving. This is known as the

A) Premack principle.
B) overjustification effect.
C) availability heuristic.
D) opponent-process theory.

Overjustification Effect

The phenomenon where providing external incentives for an already internally rewarding behavior can lead to a reduction in intrinsic motivation.

Premack Principle

A principle of reinforcement that states a more preferred activity can be used as a reward to reinforce a less preferred activity.