题目内容：
根据以下材料回答题 Tofightanoisebymakingmorenoisesoundsstrange,butthat'sexactlywhatmotorengi-neersaredoinginGermanyandsomeothercountries．
Carmaker'sresearchanddevelopmentlaboratorieshavealreadyprovedthatmixingmorenoisewiththehelpofloudspeakerscanreducetheunwantednoise．Physicistshaveknownaboutthetechniqueforalongtime．Soundismadeupofpressurewavesintheair．Iftwosoundwavesofthesamefrequency(频率)mixsothatthehighestpointofonewavehappensatthesametimeasthelowestpointoftheotherwave,theresultisnosound．Therefore,byproducingaperfectcopyofthenoiseanddelayingitbyhalfawavecycle(周期),wecankilltheunwantednoise．Usingthistechniquemanycarmakersareracingtodevelopnoisekillingsystems(系统)bothinsideandoutsidethecars．
Anothergoodthingabouttheuseofnoisekillingsystemsisthatitsavestheneedforasilencer;thisnotonlyreducestheweightofacar,butalsomakesthemotorburnlessfuelandworkbetter．
Someengineersbelievethatthenoisekillingsystemwillbeusedinmostcarswithinafewyears．Butthecarmakershaven'tdecidediftheywillcarryitintoproductionbecauseitwouldaddseveralhundreddollarstothecostoftheircars．
Whichofthefollowinggivesageneralideaofhowthenoisekillingsystemworks? A．Byproducingpressurewavesgoingintheoppositedirection．
B．Bymixinghighfrequencysoundwaveswithlowfrequencysoundwaves．
C．Bymixingnewsoundwaveswiththenoiseandsendingthemouttogether．
D．Bymakingcopiesoftheunwantedsoundwavesandlettingthemoutalittlelater．
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