懂你英语l3-unit-3

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1、Unit3-1ListeningTherearemanyformsoflivesonearth,includinghumanbeings.Lifeexistsinavarietyofconditions.Someformsofliveliveinthewateryenvironment,liketheoceans.Otherformsoflivescanbefoundinverydryareaslikedesserts.However,foranyformoflivetoexist,conditionsmustberight.Whentheconditionsaren’tri

2、ght,thatformoflivescanextinct.Tobecomeextinctmeanstodieoutcompletely.Conditionsmustberightforlifetoexist.Millionsofyearsago,therewereformsoflivesthatnolongerexist.Whenconditionchanged,thisformoflifediedout.Oneextinctioneventhappenedabout250millionyearsago.Thisisthelargestextinctioneventofal

3、ltime.Manyformsoflivesbecomeextinct.96%ofalllivesintheoceansdiedout.Mostinsectsalsobecameextinct.Thiseventhappenedoveraperiodofseveralmillionyears.Thecausesofthisextinctioneventarestillunknown.Possiblecausesincludelargestvolcaniceruptionsandglobalwarming.Somescientistsbelievethattherewerese

4、veralcauses.Theybelievethataseriesofeventscausedtheextinctions.Scientistsareworkingforbetterunderstandwhatreallyhappened.Inmoderntimes,wehumanbeingsfacechangingconditions.Forhumanstolive,weneedcleanairandcleanwater.Pollutionisnowagrowingproblemaroundtheworld.Pollutionpoisonstheairandwaterth

5、atwedependon.Pollutedairmakespeoplesickandafraidtogetoutside.Pollutedwaterpoisonsourfoodsupply.Asaresult,weneverknowwhichfoodsaresafetoeat.Humanbeingscanbenotliveinapoisonedenvironment.Therefore,pollutionisamajorthreattoourexistence.Humansneedtemperaturetobeinacomfortablerange.Tobeinacomfor

6、tablerangemeanstobeneithertoohotnortoocold.Withglobalwarming,globaltemperaturesarerising.Asglobaltemperaturesrise,thepolaricecapswillmelt.Asthepolaricecapsmelt,oceanlevelswillrise.Areasofsomecountrieswillsoonbeunderwater.Peoplewouldbeforcedtorelocatedfromfloodedareas.Innature,evensmallchang

7、escansometimeshavelargeeffects.It’sdifficulttopredictwhat’sgoingtohappen.Theentireecosystemthatwedependonischanging.Someofthesechangingsareirreversible.Irreversiblechangescannotbeundone.Let’shopethehumansaresmartenoughtounderstandhowtheworldischanging.Wi