根据短文回答 31～35 题。
Sleep Necessary for Memories
Burning the midnight oil before an exam or interview does harm to the performance according to a recent research which found that sleep is necessary for memories to be taken back into the brain.A good night's sleep within 30 hours of trying to remember a new task is a required condition of having good recall in the weeks ahead。scientists}lave found.
The research,published in the December issue of Nature Neuroscience.Showed that it was the act of sleep.rather than the simple passage of time,that was critical for long-term memory formation.
"We think that getting that first night's sleep starts the process of minify consolidation(巩固),"said Robert Stick gold,a sleep researcher at Hayward Medical! School who conducted the latest study.
"It seems that memories normally wash out of the brain unless some process nails them down.My suspicion is that sleep is one of those things that does the nailing down,Professor Stick gold said.
With about one in five people claiming that they are so chronically short of sleep that it affects their daily activities.the latest work emphasizes the less well-understood side effect—serious memory impairment(损害).
Volunteers in an experiment found it easier to remember a memory task if they were allowed to sleep that night.But for those kept awake ,no amount of subsequent sleep made up for the initial loss.
Professor Stick gold's team trained 24 people to identify the direction of three diagonal(斜线形的)bars flashed for a sixtieth of a second on a computer screen full of horizontal(水平的)stripes.
Half of the subjects were kept awake that night。while the others slept.Both groups were allowed to sleep for the second and third nights to make up for any differences in tiredness between the volunteers.
Those who slept the first night were significantly and consistently better at remembering the task while the second group showed no improvement despite enjoying two nights of catch.up sleep.
第 31 题 The research published in Nature Neuroscience showed that what was essential to the formation of long—term memory was（）

A. intelligence.
B. time.
C. food.
D. sleep.