懂你英语level3-unit3

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

2、Whenconditionsarenotright,thatformoflifewillbecomeextinct.Tobecomeextinctmeanstodieoutcompletely.-Whatmustberightforlifetoexist?-Conditions.-Whathappensifconditionsarenotright?–Lifecannotexist.Conditionsmustberightforlifetoexist.Millionsofyearsago,therewereformsoflifetha

3、tnolongerexist.Whenconditionschanged,theseformsoflifediedout.Oneextinctioneventhappenedabout250millionyearsago.Thiswasthelargestextinctioneventofalltime.Manyformsoflifebecameextinct.96%ofalllifeintheoceansdiedout.Mostinsectsalsobecameextinct.Thiseventhappenedoveraperiodo

4、fseveralmillionyears.-Whathappenedabout250millionyearsago?–Manyformsoflifebecameextinct.-Whathappensinanextinctionevent?–Manyformsoflifebecameextinct.Thecausesofthisextinctioneventarestillunknown.Possiblecausesincludelargevolcaniceruptionsandglobalwarming.Somescientistsb

5、elievethattherewereseveralcauses.Theybelievethataseriesofeventscausedtheextinctions.Scientistsareworkingtobetterunderstandwhatreallyhappened.-Whatistheonepossiblecauseofthisevent?–Volcaniceruptions.Inmoderntimes,wehumansfacechangingconditions.Forhumanstolive,weneedcleana

6、irandcleanwater.Pollutionisnowagrowingproblemaroundtheworld.Pollutionpoisonstheairandwaterthatwedependon.Pollutedairmakespeoplesickandafraidtogooutside.Pollutedwaterpoisonsourfoodsupply.Asaresult,weneverknowwhichfoodsaresafetoeat.Humanbeingscannotliveinapoisonedenvironme

7、nt.Therefore,pollutionisamajorthreattoourexistence.-Whatdohumansneedtolive?–Cleanairandcleanwater.-Whatpoisonstheairandwater?–Pollution.Humansneedtemperaturestobeinacomfortablerange.Tobeinacomfortablerangemeanstobeneithertoohotnortoocold.Withglobalwarming,globaltemperatu

8、resarerising.Astemperaturesrise,thepolaricecapswillmelt.Asthepolaricecapsmelt,oceanlevelswillrise.Areas