是一道句意改写题...
Thereisstillnouniversallyagreed-onexplanationforwhythereshouldbesuchadramaticcessationoftreegrowthattheuppertimberline.Variousenvironmentalfactorsmayplayarole.Toomuchsnow,forexample,cansmothertrees,andavalanchesandsnowcreepcandamageordestroythem.Late-lyingsnowreducestheeffectivegrowingseasontothepointwhereseedlingscannotestablishthemselves.【Windvelocityalsoincreaseswithaltitudeandmaycauseseriousstressfortrees,asismadeevidentbythedeformedshapesathighaltitudes.】Somescientistshaveproposedthatthepresenceofincreasinglevelsofultravioletlightwithelevationmayplayarole,whilebrowsingandgrazinganimalsliketheibexmaybeanothercontributingfactor.Probablythemostimportantenvironmentalfactoristemperature,forifthegrowingseasonistooshortandtemperaturesaretoolow,treeshootsandbudscannotmaturesufficientlytosurvivethewintermonths.
9.Whichofthesentencesbelowbestexpresstheessentialinformationinthehighlightedsentenceinthepassage?Incorrectchoiceschangethemeaninginimportantwaysorleaveoutessentialinformation.
①Becauseoftheirdeformedshapesathighaltitudes,treesarenotlikelytobeseriouslyharmedbythestrongwindstypicalofthosealtitudes.
②Asaltitudeincreases,thevelocityofwindsincrease,leadingtoaseriousdecreaseinthenumberoftreesfoundathighaltitudes.
③Thedeformedshapesoftreesathighaltitudesshowthatwindvelocity,whichincreasewithaltitude,cancauseserioushardshipfortrees.
④Increasedwindvelocityathighaltitudesdeformstheshapesoftrees,andthismaycauseseriousstressfortrees.
------------------------
帮我分析一下三和四吧,这两个真的想不清楚,我选了3.