In an A-B-C-B Design, What Is the "C

In an A-B-C-B design, what is the "C?"

A) it means that reinforcement is given, but it is not contingent on the target behavior being performed
B) it means that reinforcement is given, and it is made contingent on the target behavior being performed
C) this is the point in a changing criterion design where a change (C) occurs
D) this is an alternating treatment (C is one treatment, B is another, and A is the third type of treatment)