In an ABAB reversal design, the second "B" period is necessary to rule out:

A) carry-over effects.
B) correlational variables.
C) chance fluctuations.
D) baseline changes.

The second "B" phase in an ABAB reversal design is crucial for demonstrating that changes in the dependent variable are due to the manipulation of the independent variable, rather than chance fluctuations. This design allows researchers to see if reintroducing the treatment (in the second "B" phase) leads to a consistent effect, thereby ruling out chance as the cause of the observed changes.