大学英语四级模拟试题(9)

《大学英语四级模拟试题(9)》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

1、大学英语四级模拟试题(9)Directions:Therearefourreadingpassagesinthispart.Eachpassageisfollowedbysomequestions.ForeachquestiontherearefoursuggestedanswersmarkedA,B,CandD.YoushouldchoosetheonebestanswerandblackenthecorrespondingletterontheANSWERSHEETwithapencil.Passage1Howdoyousendamessagetoasubmerge

2、dsubmarine,particularlyonecarryingmiWatermaynotlooklikeabarriertocommunications,butappearancesaredeceiving.Waterstronglyabsorbsallelectromagneticwavesexceptbluegreenlightandextremelylowfrequencyradiowaves.Theverylowfrequencywavesnowusedtocontractsubmarinespenetratesonlyashortdistanceinto

3、theocean,sothecraftmusteithersurfaceorsendupandantenna(天线)toreceivemessages,therebyincreasingitsweakness.Alasersystem---accurateoverlongdistancesandcapableofcarryingmoredatathantheverylowfrequencywaves---wouldtalktosubmarineattheirnormaldepths.Inthesystem,averybroadbeamspreadingoutfreely

4、inalldirectionswouldbescanned(扫描)overthousandsofsquaremilesofoceansothatitwouldn'tendangerboats,birdsorfish——orthesubmarinesitissupposedtoreach.Sinceonlyasmallfractionofthelasersystemwillmakeitswaythroughtheairandocean,receiversmountedonthesubmarinesmustbeablenotonlytodetectthelaserbutal

5、sotodiscriminatebetweenitandsunlight.So,militaryscientistsarenowworkinghardonspecialfilterthatallowthroughonlytheprecisewavelengthsemittedbythelaser.Thefilteredlight,whentransformedintoelectricalsignal,canthenbedecoded.Militaryplannersareconfidentthatlasercommunicationwithsubmarinesisfea

6、sible.1.Whichofthefollowingdoesthepassagemainlydiscuss?A.Missilescarriedbysubmergedsubmarines.B.Messagessentbysubmergedsubmarines.C.Blue-greenlasersusedbysubmergedsubmarines.D.Thewaytosendamessagetosubmergedsubmarines.2.WhichofthefollowingisTRUEaccordingtothepassage?A.Waterisabarriertora

7、diocommunication.B.Lasershavefoundwideapplicationinsubmarinecommunication.C.Waterabsorbsallkindsofelectromagneticwaves.D.Verylowfrequencyradiowavescannotbeusedtocontactsubmarines.3.WhichofthefollowingisNOTtrueofalasersystem?A.Itisabletomakeitswaythroughwater.B.Itisabletoc