懂你英语Level 3 unit 3

Level3unit3part1ListeningTherearemanyformsoflifeonEarth,includinghumanbeings.Lifeexistsinavarietyofconditions.Someformsoflifeliveinawateryenvironment,liketheoceans.Otherformsoflifecanbefoundinverydryareas,likedeserts.However,foranyformoflifetoexist,conditionsmustberight.Whenconditionsarenotright,thatformoflifewillbecomeextinct.Tobecomeextinctmeanstodieoutcompletely.-Whatmustberightforlifetoexist?-Conditions.-Whathappensifconditionsarenotright?-Lifecannotexist.Conditionsmustberightforlifetoexist.Millionsofyearsago,therewereformsoflifethatnolongerexist.Whenconditionschanged,theseformsoflifediedout.Oneextinctioneventhappenedabout250millionyearsago.Thiswasthelargestextinctioneventofalltime.Manyformsoflifebecameextinct.96%ofalllifeintheoceansdiedout.Mostinsectsalsobecameextinct.Thiseventhappenedoveraperiodofseveralmillionyears.-Whathappenedabout250millionyearsago?-Manyformsoflifebecameextinct.-Whathappensinanextinctionevent?-Manyformsoflifebecameextinct.Thecausesofthisextinctioneventarestillunknown.Possiblecausesincludelargevolcaniceruptionsandglobalwarming.Somescientistsbelievethattherewereseveralcauses.Theybelievethataseriesofeventscausedtheextinctions.Scientistsareworkingtobetterunderstandwhatreallyhappened.-Whatistheonepossiblecauseofthisevent?-Volcaniceruptions.Inmoderntimes,wehumansfacechangingconditions.Forhumanstolive,weneedcleanairandcleanwater.Pollutionisnowagrowingproblemaroundtheworld.Pollutionpoisonstheairandwaterthatwedependon.Pollutedairmakespeoplesickandafraidtogooutside.Pollutedwaterpoisonsourfoodsupply.Asaresult,weneverknowwhichfoodsaresafetoeat.Humanbeingscannotliveinapoisonedenvironment.Therefore,pollutionisamajorthreattoourexistence.-Whatdohumansneedtolive?-Cleanairandcleanwater.-Whatpoisonstheairandwater?-Pollution.Humansneedtemperaturestobeinacomfortablerange.Tobeinacomfortablerangemeanstobeneithertoohotnortoocold.Withglobalwarming,globaltemperaturesarerising.Astemperaturesrise,thepolaricecapswillmelt.Asthepolaricecapsmelt,oceanlevelswillrise.Ar...