If you studied for this exam while you were unusually happy, you will probably do best taking it while you are:

A) unusually sad.
B) unusually happy.
C) moderately happy.
D) happy when you know the answers and sad when you don't.

This is based on the psychological principle of state-dependent learning, which suggests that individuals recall information more effectively when their physiological or emotional state at the time of recall matches the state they were in when they originally learned the information.