Which of the following is a reason why stage 4 is considered to be the deepest sleep stage?

A) A person in this stage is easily roused by meaningful stimuli.
B) A person who awakes from stage 1 is groggy and confused.
C) A person who awakes from this stage is alert and attentive.
D) Only very loud stimuli will awaken a person from stage 4 sleep.
E) Napping in stage 4 reduces the amount of sleep required per night.

Answer: D
Rationale: The fact that only very loud stimuli will awaken a person from stage 4 sleep is the reason why stage 4 is considered to be the deepest sleep stage.