olarradiationcanbeconvertedintoheatthatwilldriveelectricpowergenerators.Seriousproblemsstillremainasto(16)andstorageofsolarenergy. Geothermalenergyistheenergycontainedwithintheearth.Heatisabundantlyavailabledeepintheearth'scoreandisconstantlybeingproduced.However,thisheatisusuallylocatedattoodeepalevelfor(17)exploitation.Inshort,verylittleisknownontheuseofgeothermalenergy,andithas(18)beenexploited. Nuclearenergyisproducedinnuclearpowerplants.Attheseplantsatomsofuraniumaresplit,thus(19)massesofenergy.Anothersourceofenergy underdevelopmentisthenuclearfusionofcertainatomsofhydrogen.Thiscouldeventually(20)naturalgasasasourceofenergy.A.rarely B.transformation C.fuel D.replaceE.led F.alternative mercial H.briefly I.derived J.variousK.relieving L.releasing M.transportation N.financial O.described
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