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1、阅读理解训练(5)阅读下列四篇短文,从每小题后所给的A,B,C或D四个选项中,选出最佳选项。Riversmaybeasignificantsourceofthegreenhousegasnitrousoxide(一氧化二氮),scientistsnowfind.TheircalculationsuggeststhatacrosstheglobethewaterwayscontributethreetimestheamountofnitrousoxidetotheatmosphereashadbeenestimatedbytheInternationalPanelonCli
2、mateChange(IPCC),theUnitedNationsscientificbodychargedwithreviewingclimatechangeresearch.Theyfoundthattheamountofnitrousoxideproducedinstreamsisrelatedtohumanactivitiesthatreleasenitrogen(氮)intotheenvironment,suchasfertilizeruseandsewagedischarges.“Humanactivities,includingfossilfuelcombu
3、stionandintensiveagriculture,haveincreasedtheavailabilityofnitrogenintheenvironment,”saidJakeBeaulieuoftheUniversityofNotreDameandtheU.S.EnvironmentalProtectionAgencyinCincinnati,Ohio,andleadauthorofthepaperpublishedthisweekinthejournalProceedingsoftheNationalAcademyofSciences.“Muchofthis
4、nitrogenistransportedintoriverandstreamnetworks,”Beaulieusaid.There,microbes(微生物)convertthenitrogenintonitrousoxide(alsocalledlaughinggas)andaninertgascalleddinitrogen(二氮).Thefindingisimportant,theresearcherssay,becausenitrousoxideisapowerfulgreenhousegasthatcontributestoclimatechangeandd
5、estructionofthestratosphere’sozonelayer,whichprotectsusfromthesun’sharmfulultraviolet(紫外线)radiation.Comparedwithcarbondioxide,nitrousoxideis300-foldmorepowerfulintermsofitswarmingpotential,thoughcarbondioxideisafarmorecommongreenhousegas.Scientistsestimatenitrousoxideaccountsforabout6perc
6、entofhuman-inducedclimatechange.Beaulieuandcolleaguesmeasurednitrousoxideproductionratesin72streams.Whensummedacrosstheglobe,theresultsshowedriversandstreamsarethesourceofatleast10percentofhuman-causednitrousoxideemissionstotheatmosphere.-8-“Changesinagriculturalandland-usepracticesthatre
7、sultinlessnitrogenbeingdeliveredtostreamswouldreducenitrousoxideemissionsfromrivernetworks,”Beaulieusaid.1.Fromthesecondparagraphwecanlearn .A.actuallyriversgiveoffmuchmorenitrousoxidethanexpectedB.scientists’calculationistotallywrongC.humanactivitiesreleasenitrou