Duke Is Exhausted and Plops Down on the Couch

Duke is exhausted and plops down on the couch. Fifteen minutes after falling asleep, Duke is not awakened by the refrigerator cycling on. He is, however, awakened by his roommate opening the refrigerator door. Duke is most likely in

A) stage 1 N-REM.
B) stage 2 N-REM.
C) stage 3 or 4 N-REM.
D) REM sleep.