单选题NorthAmericanGrasslandsInNorthAmerica,nativegrasslandsoccurprimarilyintheGreatPlainsinthemiddleofthecontinent.TheNorthAmericanprairiebiomeisoneofthemostextensivegrasslandsintheworld,extendingfromtheedgeoftheRockyMountainsinthewesttothedeciduousforestintheeast,andfromnorthernMexicointhesouthtoCanadainthenorth.Averageannualrainfallrangesfromabout40cm(16inches)inthewestto80cm(31inches)intheeast.Averageannualtemperaturesrangebetween10degreesand20degreesCelsius(50to68degreesFahrenheit).InthemoistregionsoftheNorthAmericangrasslands,especiallyinthenorthernGreatPlains,rainfallisdistinctlyseasonal,andtemperaturescanvarywidelyfromveryhotinsummertobittercoldinwinter.Onehundredyearsago,theGreatPlainsgrasslandswereonevast,unbrokenprairie.Muchoftheprairieisnowfarmland,themostproductiveagriculturalregionintheworld,dominatedbymonoculturesofcerealgrains.Wheat,barley,soybeans,corn,andsunflowersoccupythelandthatwasonceprairie.Inareasgivenovertograzinglandsforcattleandsheep,virtuallyallthemajornativegrasseshavebeenreplacedbyalienspecies.AnimportantfeatureofthenorthernGreatPlainsgrasslandsisthepresenceofmillionsofglacialdepressionsthatarenowsmallpondsknownasprairiepotholes.TheywereformedduringthemostrecentIceAge,whenstreamsflowedintunnelsbeneathglaciallyformedsandyridges.WhentheIceAgeendedaround12,000yearsago,theretreatingglacierscreatedabout25milliondepressionsacrossa300,000-square-milelandscape--about83potholespersquaremile.Astheiceblocksmelted,muchofthewaterwasleftbehind,formingwetlandsranginginsizefromatenthofanacretoseveralacres.Thewetlandsweresoonsurroundedbyflutteringwavesofgrasses:shortgrass,mixedgrass,andtallgrass.Todaythesesmallwetlandsstillcovertheprairies,althoughmuchofthelandscapeincludingbothnativegrassesandpotholeshasbeentransformedtocroplandandgrasslandforgrazing.Whatdoesremainofthewetlands,however,stillservesasanimportantbreedingareaformorethan300birdspecies,includinglargenumbersofmigratingshorebirdsandwaterfowl.Thepotholesfillupwithwaterduringspringrainsandusuallydryoutbylatesummer.Everyspring,birdsarriveingreatnumbers--northernpintails,mallards,coots,andpied-billedgrebes--4to6millionstrong,tomateintheseasonalwetlandsthatdotportionsofMinnesota,Iowa,NorthandSouthDakota,Montana,Alberta,Saskatchewan,andManitoba.PrairiepotholecountryproduceshalfofNorthAmerica's35to40millionducksandisrenownedworldwideasa"duckfactory".Recentlybiologistshavediscoveredthattheprairiepotholeregionispotentiallyavastcarbonsink:anaturalspongethatabsorbscarbondioxideemissionsfromcars,factories,andpowerplants.Carbondioxideisthemostcommonofallthepollutantsactingasgreenhousegasesthatheatuptheatmosphere.Fortunately,however,carbondioxideiscapturednaturallyandstoredintrees,soil,andplants.Scientistshavetermedthis"carbonsequestration".Theyhavedeterminedthatprairiepotholesholdanaverageof2.5tonsofcarbonperacreperyearwhennotbeingfarmed.ThismeansthatiftheentirepotholeregionintheUnitedStatesandCanadaweretostopbeingfarmed,theregionwouldstoreabout400milliontonsofcarbonover10years--theequivalentoftakingalmost4millioncarsofftheroad.Thus,preservingthepotholescouldbeawaytooffsetgreenhousegasemissionsthatarewarmingtheplanet.Glossary:biome:oneoftheworld'smajornaturalcommunities,classifiedbypredominantvegetationmonoculture:cultivationoflargelandareaswithasingleplantvariety

单选题Some of sharpshooter Annie Oakley’s exploits with a gun are almost unbelievable when it comes to accuracy, speed of firing,and endure.

单选题Made of hard wood, the boomerang is roughly V-shaped, with arms — skewed

单选题Most geologists believe — from the remains of tiny marine plants and animals that died millions of years ago.

单选题Hydrofoils can exceed 75 miles per hour, as compared with conventional watercraft, whose maximum speeds .......... approach 50 miles per hour.

单选题The bathyscaphe, a free-moving vessel designed for underwater exploration, consists of a Flotation compartment with a observation capsule attached underneath it.

单选题Listening2"LiteratureClass"

单选题1 Erik Erikson believed that personality development is a series of turning points, which he described in terms of the tension between desirable qualities and dangers. He emphasized that only when the positive qualities outweigh the dangers does healthy psychosocial development take place. 2 An important turning point occurs around age six. A child entering school is at a point in development when behavior is dominated by intellectual curiosity and performance. He or she now learns to win recognition by producing things. The child develops a sense of industry. The danger at this stage is that the child may experience feelings of inadequacy or inferiority. If the child is encouraged to make and do things, allowed to finish tasks, and praised for trying, a sense of industry is the result. On the other hand, if the child's efforts are unsuccessful, or if they are criticized or treated as bothersome, a sense of inferiority is the result. For these reasons, Erikson called the period from age six to eleven Industry vs. Inferiority.

单选题Listening6"HistoryClass"

单选题Although most petroleum is produced from underground reservoirs, petroleum occurs in a varieties of forms at the surface

单选题Connecticut was the fifth of the original thirteen states ........ the Constitution of the United States.

单选题Listentopartofaconversationbetweenapart-timestudentandthemanageroftheschoolcafeteria.Nowgetreadytoanswerthequestions.Youmayuseyournotestohelpyouanswer.

单选题The round, hard-shelled nuts of the macadamia tree grow in clusters and are covered with husks what split when the nuts are ripe

单选题Mr. Hui, a freelance writer here at ESS Newspapers, has impeccable for the position of senior editor.

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单选题Why does the author mention "aragonite" in line 26

单选题Stagecoaches reached their greatest importance in the United Slates in the nineteenth century, when paved roads made travel —-~.

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