Edgar cannot sleep because he is terribly worried about his research paper. So, Edgar decides to get out of bed and continue working on the paper. Although he stays up to nearly 3 a.m., he is relieved that it is done and easily falls off to sleep. In the future, Edgar will be more likely to finish his work before going to bed so that he can avoid the worry and sleeplessness. Such behavior is an example of

a. negative reinforcement.

b. positive reinforcement.

c. punishment.

d. classical conditioning.