Parents Are Interested in Getting Their Son to Play Piano

Parents are interested in getting their son to play piano. In order to do this, they decide to reinforce him by paying him $1.00 for every hour that he practices. Shortly after this, the son decides that he also wants to learn how to play guitar and, since his parents are still paying him, he continues practicing both instruments. After a couple of months, the parents decide that the important thing is that their son is involved in music and so they quit paying him to practice the piano, whereupon the son gradually quits playing the
Piano and continues playing the guitar. The weakening and disappearance of the son's piano-playing behaviour would best be considered as an example of:

A) positive reinforcement.
B) negative reinforcement.
C) classical extinction.
D) operant extinction.