[简答]  【完形填空题】Choosethebestanswerforeachblank tocompletethefollowingpassages.SavingtheEarth   Thereare waysinwhichpeopletrytodealwiththeproblemofenergy.Oneway isthegreaterproductionofcommonenergysources,suchascoal,oiland.The withthesesources,however,isthattheyarenotrenewable.   Anotherwayisenergyconservation,whichmeansusingenergymore.Insomeverycoldcountriespeoplebuildspecialhousestosaveenergy.Theyplacematerialsbetweenthe insideandtheoutsideofthewallsofthehousetokeepthecoldoutandthewarmth in.Thehouseisheatedbythelights,thebodyheatofthepeopleandtheotherequipmentinit.   Finally,renewableenergysourcesareusedeventhoughtheyareoftenexpensivetodevelop.Oneformoftheseisgeothermalenergy.Incertainpartsoftheworldthe oftheearthincreasesthirtydegreescentigradewitheachkilometerdown.Atsixkilometers,therefore,itrisestonearlytwohundreddegrees.Togettheheat,waterispumpeddownintotherocksandbackuptothesurface.Heatfromtheearthisalreadyusedincertaincountries.