懂你英语l3 unit 3

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1、Unit3-1ListeningTherearemanyformsoflivesonearth,includinghumanbeings.Lifeexistsinavarietyofconditions.Someformsofliveliveinthewateryenvironment,liketheoceans.Otherformsoflivescanbefoundinverydryareaslikedesserts.However,foranyformoflivetoexist,conditionsmu

2、stberight.Whentheconditionsaren’tright,thatformoflivescanextinct.Tobecomeextinctmeanstodieoutcompletely.Conditionsmustberightforlifetoexist.Millionsofyearsago,therewereformsoflivesthatnolongerexist.Whenconditionchanged,thisformoflifediedout.Oneextinctionev

3、enthappenedabout250millionyearsago.Thisisthelargestextinctioneventofalltime.Manyformsoflivesbecomeextinct.96%ofalllivesintheoceansdiedout.Mostinsectsalsobecameextinct.Thiseventhappenedoveraperiodofseveralmillionyears.Thecausesofthisextinctioneventarestillu

4、nknown.Possiblecausesincludelargestvolcaniceruptionsandglobalwarming.Somescientistsbelievethattherewereseveralcauses.Theybelievethataseriesofeventscausedtheextinctions.Scientistsareworkingforbetterunderstandwhatreallyhappened.Inmoderntimes,wehumanbeingsfac

5、echangingconditions.Forhumanstolive,weneedcleanairandcleanwater.Pollutionisnowagrowingproblemaroundtheworld.Pollutionpoisonstheairandwaterthatwedependon.Pollutedairmakespeoplesickandafraidtogetoutside.Pollutedwaterpoisonsourfoodsupply.Asaresult,weneverknow

6、whichfoodsaresafetoeat.Humanbeingscanbenotliveinapoisonedenvironment.Therefore,pollutionisamajorthreattoourexistence.Humansneedtemperaturetobeinacomfortablerange.Tobeinacomfortablerangemeanstobeneithertoohotnortoocold.Withglobalwarming,globaltemperaturesar

7、erising.Asglobaltemperaturesrise,thepolaricecapswillmelt.Asthepolaricecapsmelt,oceanlevelswillrise.Areasofsomecountrieswillsoonbeunderwater.Peoplewouldbeforcedtorelocatedfromfloodedareas.Innature,evensmallchangescansometimeshavelargeeffects.It’sdifficultto

8、predictwhat’sgoingtohappen.Theentireecosystemthatwedependonischanging.Someofthesechangingsareirreversible.Irreversiblechangescannotbeundone.Let’shopethehumansaresmartenoughtounderstandhowtheworldischanging.Wi