懂你英语level 3 unit 3

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1、Level3unit3part1ListeningTherearemanyformsoflifeonEarth,includinghumanbeings.Lifeexistsinavarietyofconditions.Someformsoflifeliveinawateryenvironment,liketheoceans.Otherformsoflifecanbefoundinverydryareas,likedeserts.However,foranyformoflifetoexist,conditionsmustberight.Whencondition

2、sarenotright,thatformoflifewillbecomeextinct.Tobecomeextinctmeanstodieoutcompletely.-Whatmustberightforlifetoexist?-Conditions.-Whathappensifconditionsarenotright?–Lifecannotexist.Conditionsmustberightforlifetoexist.Millionsofyearsago,therewereformsoflifethatnolongerexist.Whenconditi

3、onschanged,theseformsoflifediedout.Oneextinctioneventhappenedabout250millionyearsago.Thiswasthelargestextinctioneventofalltime.Manyformsoflifebecameextinct.96%ofalllifeintheoceansdiedout.Mostinsectsalsobecameextinct.Thiseventhappenedoveraperiodofseveralmillionyears.-Whathappenedabout

4、250millionyearsago?–Manyformsoflifebecameextinct.-Whathappensinanextinctionevent?–Manyformsoflifebecameextinct.Thecausesofthisextinctioneventarestillunknown.Possiblecausesincludelargevolcaniceruptionsandglobalwarming.Somescientistsbelievethattherewereseveralcauses.Theybelievethataser

5、iesofeventscausedtheextinctions.Scientistsareworkingtobetterunderstandwhatreallyhappened.-Whatistheonepossiblecauseofthisevent?–Volcaniceruptions.Inmoderntimes,wehumansfacechangingconditions.Forhumanstolive,weneedcleanairandcleanwater.Pollutionisnowagrowingproblemaroundtheworld.Pollu

6、tionpoisonstheairandwaterthatwedependon.Pollutedairmakespeoplesickandafraidtogooutside.Pollutedwaterpoisonsourfoodsupply.Asaresult,weneverknowwhichfoodsaresafetoeat.Humanbeingscannotliveinapoisonedenvironment.Therefore,pollutionisamajorthreattoourexistence.-Whatdohumansneedtolive?–Cl

7、eanairandcleanwater.-Whatpoisonstheairandwater?–Pollution.Humansneedtemperaturestobeinacomfortablerange.Tobeinacomfortablerangemeanstobeneithertoohotnortoocold.Withglobalwarming,globaltemperaturesarerising.Astemperaturesrise,thepolaricecapswillmelt.Asthepolaricecapsmelt,oceanlevelswi

8、llrise.Areas