[单项选择]Before a big exam, a sound night’s sleep will do you more good than poring over textbooks. That, at least, is the folk wisdom. And science, in the form of behavioral psychology, supports that wisdom. But such behavioral studies cannot distinguish between two competing theories of why sleep is good for memory. One says that sleep is when permanent memories form. The other says that they are actually formed during the day, but then "edited" at night, to flush away what is superfluous. To tell the difference, it is necessary to look into the brain of a sleeping person, and that is hard. But after a decade of painstaking work, a team led by Pierre Maquet at Liege University in Belgium has managed to do it. The particular stage of sleep in which the Belgian group is interested is rapid eye movement (REM) sleep, when brain and body are active, heart rate and blood pressure increase, the eyes move back and forth behind the eyelids as if watching a movie, and brainwave traces resembl
A. the brain works more efficiently by knowing a set pattern of things
B. the second theory failed to cover all the brain responses during sleep
C. REM sleep reactivates connections between the nerves and the memory
D. it’s beyond doubt that the subjects were learning in contrast to unlearning