As a marine biologist, you are trying to teach a dolphin to jump over a bar. At first, you reward the dolphin every time it swims near the bar. Then, you only reward her when she emerges from the water near the bar. Eventually, you reward the dolphin each time she jumps out of the water. Then, you only reward the dolphin when she jumps over the bar. This technique is an example of
A. classical conditioning.
B. spontaneous recovery.
C. discrimination.
D. shaping.
E. positive punishment.