根据短文内容，从短文后的七个选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

  Every animal sleeps,but the reason for thishas remained foggy.When lab rats are not allowed to sleep,they die within amonth.    71    

    One idea is that sleep helps us strengthennew memories.    72   We know that,while awake,fresh memories are recorded by reinforeing (加强)connections between braincells,but the  memory processes that takeplace while we sleep  have been unclear.

    Support is  growing for a theory thatsleep evolved so that connections between neurons(神经元)in the brain can be weakenedovernight,making room for fresh memories to from the next day.       73  

Now we have the most direct evidence yet that he is right.   74   Thesynapses in the  mice taken at the end ofa period of sleep were 18 per cent smaller than those taken beforesleep,showing that the connections between neurons weaken  while sleeping.

If  Tononi`s theory isright,it would explain why,when we miss a night`s,we find it harder the nextday to concentrate and learn new information-our brains may have smaller roomfor new experences.

Their research also suggests how we may build lasting memories overtime even though the synapscs become thinner.The team discovered that somesynapses seem to be protected and stayed the same size.   75   “You keep what matters,”Tononi says.

A. We should alsotry to sleep well the night before.

B. Ti’s as if thebrain is preserving its most important memories.

C. Similarly, whenpeople go for a few days without sleeping, they get sick.

D. The processestake place to stop our brains becoming loaded with memories.

E. That’s whystudents do better in tests if they get a chance to sleep after learning.

F. “Sleep is theprice we pay for learning,” says Giulio Tononi, who developed the idea.

G. Tononi’s teammeasured the size of these connections, or synapses, in the brains of 12 mice.