阅读理解练习Underproperconditions,soundwaveswillbereflectedfromahillsideorothersuchobstruction(障碍).Soundtravelsattherateofaboutone-fifthofamilepersecond.Ifthehilliselevenhundredfeetaway,ittakestwosecondsforthesoundtotraveltothehillandback.Thus,bytimingtheintervalbetweenasoundanditsreflection,youcanestimatethedistancetoanobstruction.DuringWorldWarIItheBritishusedapracticalapplication(应用)ofthislawtodetectGermanplanesontheirwaytobombLondonlongbeforetheenemywasnearthetarget.Theyusedradiowavesinsteadofsoundwaves,sinceradiowavescanmakeawaythroughfogandclouds.TheoutnumberedRoyalAirForcealwaysseemedtothepuzzledGermanstobesurprised.Itwasradioechoes(回声)morethatanythingelsethatwontheBattleofBritain.SincetheradiowaveswereusedtotellthedirectioninwhichtosendtheRAFplanesandthedistancetosendthem(theirlineofflight,inotherwords),thedevicewascalledradiodirectionandranging,andfromtheinitialsthewordradarwasinvented.1.Soundwavesreflectedfromahillcanbeusedtocalculatethe_____.A.heightofthehillB.speedofsoundC.distancetothehillD.intensityofsound2.RadarenabledtheEnglishto______.A.preventGermanplanesbeingontheirmaytoLondonB.directtheoutnumberedRAFplaneseffectivelyC.confuseGermanbomberpilotsD.numbertheRoyalAirForce3.TheBritishusedradiowavesbecausethey______.A.weremoreexactthansoundwavesB.couldnotbedetectedC.weremoreusefulthatsoundwavesD.wereeasiertousethansoundwaves4.Theauthorofthisarticleprobablyintendedtoexplain______.A.exactlyhowradarworksB.whytheBritishusedradiowavesintheirdeviceC.Howradar(wordanddevice)CametobeD.HowradarhelpedtheBritishwintheBattleofBritainKeys:1.C2.B3.C4.C
