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1、1.Nooneknows_______howtheearthbegan,___ithappenedsolongago.2.Afterthat,atomsbegantoformand________tocreatestarsandotherbodies.3.Forseveralbillionyearsafterthe‘BigBang’,theearthwasstilljust____________dust.4.Theearthbecamesoviolentthatitwasnotclearwhetherthe_
2、_______wouldlastornot.5.Theywere_________toproducecarbon,nitrogen,watervapourandother______,whichweretomaketheearth’s______________.6.Whatisevenmoreimportantisthatastheearth______________,waterbegantoappearonitssurface.7.Itwasn’timmediatelyobviousthatwaterwa
3、stobe_____________tothedevelopmentoflife.8.Thisproduceda_______reaction,whichmadeitpossibleforlifetodevelop.9.Manymillionsofyearslater,thefirst_______smallplantsbegantoappearonthesurfaceofthewater.10.They_______andfilledtheoceansandseaswithoxygen,which______
4、___thelaterdevelopmentofearlyshellfishandallsortsoffish.11.Laterwhentheplantsgrewintoforests,______appearedforthefirsttime.12.Theyproducedyoung________bylayingeggs.13.Dinosaurslaideggstooand_________ontheearthformorethan140millionyears.14.Whytheysuddenlydisa
5、ppearedstillremainsa__________.15.Thisdisappearancemadepossible__________mammalsontheearth.16.Sowhetherlifewillcontinueontheearthformillionsofyearstocomewill_______whetherthisproblemcanbesolved.17.Goingeastward,you'llpassmountainsandthousandsoflakesandforest
6、s,__________wideriversandlargecities.18.SomepeoplehavetheideathatyoucancrossCanada___________fivedays,buttheyforgetthefactthatCanadais5,500kilometersfromcoasttocoast.19.Itspopulationisincreasing____________.20.Itissowettherethatthetreesareextremelytall,some_
7、________over90meters.21.Cowboysfromallovertheworldcometo_________intheStampede.22.Manyofthemhave_________ridingwildhorsesandcanwinthousandsofdollarsinprizes.23.Theywentthroughawheat-growing___________andsawfarmsthatcoveredthousandsofacres.24.Thegirlsweresurp
8、rised___________oceanshipscansailuptheGreatLakes.25.BecauseoftheGreatLakes,theylearned,Canadahasmore_________thananyothercountryintheworld.26.Ithasone-thirdoftheworld'stotalfreshwater,and_______
