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完形填空（共20小题：每小题1.5分，满分30分）阅读下面短文，从短文后各题所给的四个选项（A、B、C、D）中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑Do you know how much your children sleep? According to the National Sleep Foundation, 41 should sleep twelve to eighteen hours 42 of every twenty-four. With a gradual 43 to twelve to fourteen hours for toddlers(初学走路的孩子) one to three; eleven to thirteen hours for 4 three to five; and ten to eleven hours for schoolchildren 45 five to ten. But things get really 46 at adolescence. Not only do teenagers need more sleep than 47 __---eight and a half to nine and a quarter hours a night, according to the sleep foundation - but the times 48 which they get sleepy and are able to awaken 49 and feel rested shift in a 50 that does not match to the 51 times at most schools. Sleep studies have shown that the typical teenager does not fall asleep readily before 11 p. m. or later. 52 many have to get up by 6 a. m. or | 
