Which statement about B. F. Skinner is correct?

A. He is famous for demonstrating the principles of operant conditioning: The motivation for a behavior happens after the behavior is demonstrated.
B. He placed hungry rats in a maze with no reward for finding their way through it, and he studied a comparison group that was rewarded with food at the end of the maze.
C. He worked with Edward C. Tolman to prove that fear is both an instinct and a reflex.
D. His experiments demonstrated that organisms can learn even if they do not receive immediate reinforcement.