What is a concern when using an experimental design that compares an "active" stage to one at "rest"?

A) If a person is unable to sleep, it is impossible to achieve a resting state
B) Individuals that exercise regularly will have less difficulty in the "active" requirements
C) The brain is not truly "at rest" in the absence of an experimental task, since people are likely to be thinking of other things
D) What one person considers to be active, another may regard as restful