高级英语视听说1 本文+答案

1Napoleonaaaaa,–––2theseaaaaa–3aa–aa3a2a2aa22a5a6aa–4aaaAaaaaaaaaa,4aa5IaaathaaaaaaaaIaaaeaaItaAyAa7AAa.AaAa.aaaAaaaaaa8IatthaaaaaaIaaIIaIIIIaI“IyIaIIaaaaaaaaafaaaAaaa(aaaa,aaaaaaAaaaaaaAaaaaaaaaaaaaaaa2aII222a…2.aaaaa22aaa2aa2–aAaaaaaaaaaSeveraltheorieshavebeenproposedaboutwhythedinosaursdisappearedfromthefaceoftheearth.Inrecentyearsonepopulartheoryproposesthatclimaticchangescausedthedinosaurstobecomeextinct.Thisclimaticchangetheorysaysthatmillionsofyearsagotheclimateoftheworldgraduallybecamecolder.Astheearthslowlybecamecolder,fewerplantswereabletogrow.Thecoldweatherfinallyresultedinasevereshortageoffoodforthedinosaurs.Asyouprobablyknow,mostofthedinosaurswerevegetarians,andtheydependedonplantsfortheirfoodsupply.Insummary,then,thedisappearanceofthedinosaurswascauseddirectlybyashortageofplantstoeat,andindirectlybyachangeintheclimate.Manyscientistsstillbelievethattheclimaticchangetheorybestexplainswhythedinosaursdisappeared.Thistheoryarguesthatthedinosaursdisappearedgradually—slowly—astheearthbecamecolderandasstheirfoodsupplydwindled.Todaythereisnewevidenceforthetheorythatthedinosaursdidnotdisappeargradually,butthattheydisappearedquicklyandsuddenly.Thistheoryisknownastheasteroidtheory.Itstatesthatahugeasteroid,orperhapsacomet,hittheearthabout65millionyearsago.Whenthiscometorasteroidhittheearth,itcausedahugedustcloud.Thehugedustcloudcoveredthewholeearthandblockedoutthesunformonths.Sincetherewasnosunformany,manymonths,mostoftheplantsonearthdiedthedinosaur’sfoodsupplywasdestroyedinaperiodofmonths.Whilethisasteroidtheoryisnotnew,whatisnewistheevidenceforthetheory.Untilrecentlytherewasnoevidencethatanasteroidoracomethadhittheearth65millionyearsago.Whathappenedrecentlywasthatscientistsfoundlargeamountsofarareearthelementcallediridiumallovertheworld.Thisiridiumwasfoundinlayersoftheearththatare65millionyearsold.Theiridiumwasfoundinthesamelayerswherethebonesofthelastdinosaurswerefound.Theelementiridiumisveryuncommon,infact,rare,ontheearth.Itisanelement,however,thatismoreoftenfoundinspace.Scientistsspeculatethatthisiridiumwasbroughttoearth65millionyearsagowhenacometorasteroidhittheearth.Thecometorasteroidtheoryexplainstwothings:(1)Itexplainsthelargeramountsoftherareelementiridiumfoundinthe65-million-year-oldlayersofearth,and(2)itexplainswhythedinosaursdisappearedfromtheearth.Todayscientistscontinuetodebatethetwotheories:theclimaticchangetheoryandtheasteroidtheory.Inthefutureevidencemaybefoundthatsupportsanewtheoryofwhytheterriblelizardsdiedout.aaIaaa42