阅读理解。

Nooneknowswhywesleepbutit'scertainthatweneedto.Peoplewhoarepreventedfromsleepingbegintosufferobviouseffectsafterafewdays----theythinklessclearlyandtheyfallasleepduringtheworkinghours;somemayhavehallucinations(幻觉).

Therearenorulesaboutsleep.Generallyspeakinggrown-upssleepabout7andhalfhourseachnightandprobablymorethan60percentgetbetweensevenandeighthours.Butperhapseightpercentarequitehappywith5hoursorlessandfourpercentorsofindthattheywanttenhoursormore.Ifyoufeelallrightyou'reprobablygettingenoughsleep.Theimportantthingisnottoworryhowmuchotherpeopleget----theirneedsmaybedifferent.Exercisedoesn'tseemtoincreasetheneedforsleep----officeworkersforexamplesleepforaboutaslongaspeopledoingphysicallyactivework.

Childrensleepmorethangrown-ups----perhaps14to18hourssoonafterbirthgoingdowntogrown-uplevelsbyearlyteen-age.Sleeppatternsalsotendtobedifferentintheelderlywhomaysleeplessatnightthantheydidwhenyoungerfindsleepgettingmorebrokenandoftenmakeitaruletosleepduringthedaytime.

1．Accordingtothepassagesomepeopleareunabletothinkclearlybecause________.

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A．theyhavehallucinations

B．theyfeelsleepyduringtheworkinghours

C．theydon'thaveenoughsleep

D．theyarecertaintobekeptfromgoingtobed

2．Whichofthefollowingistrue?________.

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A．Allgrown-upsmusthaveatleasteight-hoursleep

B．Mostofgrown-upssleepforsevenoreighthours

C．Quiteafewpeopleneedonly5hoursorlessforthemtosleep

D．Nogrown-upssleepmorethantenhours

3．Whetheryouhavegotenoughsleepisjudgedby________.

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A．howmanyhoursyouhaveslept

B．howmanyhoursyouneedtosleep

C．whetheryoudoexerciseandphysicalwork

D．whetheryoufeelfreshandenergetic

4．Accordingtothepassageaboyof14yearsoldsleeps________.

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A．aslonghoursasagrown-up

B．muchlongerhoursthanagrown-up

C．for14hourseachnight

D．forlessthan8hourspernight