A mute mental patient is first given a token each time he or she says a word. Next, tokens are given for speaking a complete sentence. Later, the patient is gradually required to speak more often, then to answer questions, and eventually to carry on a short conversation in order to receive tokens. This gradual improvement of the patient's communication skills is an example of

a. shaping.
b. reciprocal inhibition.
c. classical conditioning.
d. systematic desensitization.

A