根据以下材料，回答 Thereislittledebateaboutthefirstdevelopmentoftheenergysystemofthe22ndcentury--thatmostoftheenergyproducedonalargescalewillbeintheformofelectricpower.Thus,mostpresentconceptsformajorlong-termenergysources(solar,geothermal,fusion)arebaseduponelectricpowerproductionatcentralplants. Thesecondmajordevelopmentisthepotentialuseofhydrogenasbothabasicfueltoreplacenaturalgasandaspartofageneralsystemtoconvert,storeandtransmitelectricenergyoverlongdistrictbyfuelcells,oritcouldbeuseddirectlyasafuelforcommercial,industrialortransportationpurposes. Althoughhydrogenseemslikelytoplayamajorroleinthefutureasaformofenergytransportandstorage,otherprocessesmayalsobeuseful.Herewenoteonlytwopossibilities. Flywheels:Itappearstobeworkabletoconstructflywheelsofalmostanyneededsizethatwouldbeefficientconverters(93-95percent)infirststoringandthenreleasingelectricpower.Inaddition,thekinetic(动力的)energyofrotationiseasilyconvertedtoalternatingelectricpower.Buttheengineeringisatanearlystageofdevelopment,andadecadeormoremaypassbeforethispotentialcanbereliablyestimated. Batteriesandfuelcells:someofthemanybatteriesbeingdevelopedtodaylookquitepromising.Withinadecadeortwo,abattery-operatedelectricautomightevenbecomecompetitivewithtoday′svehicles,atleastfordrivingrangesupto200miles.Anelectricautomobilewouldbepollution-free,andwouldprobablybemoreenergy-efficientthantheinternal-combustion(内燃)engine. Afuelcell(whichmaybeconsideredahydridebattery)withabout40percentconversionefficiencyisexpectedtoenterpowerproductiononalargescalewithinaboutfiveyears.Unlikesteam-poweredgenerators,fuelcellscanbemadelargeorsmallandtheirefficiencyinoperationisessentiallyindependentoftheloading.Thustheymayavoidmostofthedistributioncostsofelectricpoweraswellasprovidepollution-fleepowerandofferopportunitiesforusingthe"wasteheat"effectively. Theenergyweusemosttodayis___________
A.solar energy B.geothermal energy C.hydrogen D.electricity正确答案D
