One Concern with Interpreting FMRI Studies Employing a "Resting State

One concern with interpreting fMRI studies employing a "resting state" for comparison with cognitive activities of interest is that

A) the brain uses considerably more blood and oxygen when it is "at rest."
B) many processes are engaged "at rest," including self-referential processes.
C) participants in experiments are particularly prone to movement artifacts when "at rest."
D) there is too little blood flow in the brain "at rest" for accurate measurements.