A math teacher displays a word problem on the overhead, asks, "What is the first thing we should do in this problem?" pauses briefly, and then says, "Calvin?" as she calls on one of the students in his class. This brief episode best illustrates which of the following?
a. A teacher's demonstration of enthusiasm.
b. A teacher's demonstration of modeling.
c. A teacher's demonstration of wait-time.
d. A teacher's demonstration of differential expectations.