The students was just about to ______ the question, when suddenly he found the answer.

A．Getting off to work with a minimum effort helps save one's energy.
B．Dr. Kleitman explains why people reach their peaks at different hour of day.
C．Habit helps one adapt to his own energy cycle.
D．Children have energy cycles, too.

A．he is a lazy person
B．he refuses to follow his own energy cycle
C．he is not sure when his energy is low
D．he is at his peak in the afternoon or evening