Andrew was a troublesome child and never did his homework. In an effort to encourage Andrew's interest in studies, his mother decided to give him a treat of his favorite chocolate whenever he completed his homework. After a few days, she noticed that Andrew started doing his homework regularly and did not even have to be reminded. This type of learning is an example of

A. respondent conditioning.
B. operant conditioning.
C. classical conditioning.
D. Pavlovian conditioning.