题目内容：
根据以下材料，回答题
HomeHeating
CentralheatingbecamepopularonlyaftertheCivilWar．Typically,coal-burningfurnaces(火炉)fueledtheearlysystems．Naturalgashaddevelopedintotheleadingfuelby1960．Itsacceptanceresultedinpartfromitswideuses．BecauseitcomesprimarilyfromU.S．andCanadianfields,naturalgasisalsolessvulnerable(脆弱的)thanoilistoWar．Oilremainsthemostimportantfuelinafewareas,suchasNewEngland.
Electricheatingdominatesmostareaswithmildwintersandcheapelectricity,includingtheSouthandtheNorthwest．ItwasmadepopularatleastintheSouthbythelowcostofaddingelectricheatingtonewhousesbuiltwithair-conditioning．Bottledgas,whichissomewhatmoreexpensivethanutilitygas,isthefuelofchoiceinruralareasnotservedbyutilitypipelines．Woodistheleadingheatingfuelinjustafewruralcounties.
Homeheating,whichaccountsforlessthan7percentofallenergyconsumedintheU.S.,hashadacommendable(值得赞扬的)efficiencyrecord:from1978to1997,theamountoffuelconsumedforthispurposedeclinedby44percentdespitea33percentincreaseinthenumberofhousingunitsandanincreaseinhousesize．TheU.S．DepartmentofEnergy,however,forecaststhatenergyusedinhomeheatingwillriseby14percentoverthenexttwodecades．Thatriseissmallconsideringanexpected21percentincreaseinthenumberofhousesandthetrendtowardlargerhouses.
Naturalgasandelectricitywillprobablydominatethehomeheatingmarketforthenexttwodecades．Solar(太阳的)heatingneverbecamepopularbecauseofcostandlimitedwintersunlightinmostareas;in2000only47,000homesreliedonit.
Naturalgasdidn'tbecometheleadingfueluntil__________. A．1978
B．1960
C．1997
D．2000
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