懂你英语lv3-unit-3

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

2、econditionsaren’tright,thatformoflivescanextinct.Tobecomeextinctmeanstodieoutcompletely.Conditionsmustberightforlifetoexist.Millionsofyearsago,therewereformsoflivesthatnolongerexist.Whenconditionchanged,thisformoflifediedout.Oneextinctioneventhappenedabout250millionyearsa

3、go.Thisisthelargestextinctioneventofalltime.Manyformsoflivesbecomeextinct.96%ofalllivesintheoceansdiedout.Mostinsectsalsobecameextinct.Thiseventhappenedoveraperiodofseveralmillionyears.Thecausesofthisextinctioneventarestillunknown.Possiblecausesincludelargestvolcanicerupt

4、ionsandglobalwarming.Somescientistsbelievethattherewereseveralcauses.Theybelievethataseriesofeventscausedtheextinctions.Scientistsareworkingforbetterunderstandwhatreallyhappened.Inmoderntimes,wehumanbeingsfacechangingconditions.Forhumanstolive,weneedcleanairandcleanwater.

5、Pollutionisnowagrowingproblemaroundtheworld.Pollutionpoisonstheairandwaterthatwedependon.Pollutedairmakespeoplesickandafraidtogetoutside.Pollutedwaterpoisonsourfoodsupply.Asaresult,weneverknowwhichfoodsaresafetoeat.Humanbeingscanbenotliveinapoisonedenvironment.Therefore,p

6、ollutionisamajorthreattoourexistence.Humansneedtemperaturetobeinacomfortablerange.Tobeinacomfortablerangemeanstobeneithertoohotnortoocold.Withglobalwarming,globaltemperaturesarerising.Asglobaltemperaturesrise,thepolaricecapswillmelt.Asthepolaricecapsmelt,oceanlevelswillri

7、se.Areasofsomecountrieswillsoonbeunderwater.Peoplewouldbeforcedtorelocatedfromfloodedareas.Innature,evensmallchangescansometimeshavelargeeffects.It’sdifficulttopredictwhat’sgoingtohappen.Theentireecosystemthatwedependonischanging.Someofthesechangingsareirreversible.Irreve

8、rsiblechangescannotbeundone.Let’shopethehumansaresmartenoughtounderstandhowtheworldischanging.Wi