托福考试真题练习_1.docx

托福考试真题练习2017年托福考试真题练习脚跟立定以后，你必须拿你的力量和技能，自己奋斗。以下是小编为大家搜整理的2017年托福考试真题练习，希望能给大家带来帮助!更多精彩内容请及时关注我们应届毕业生考试网!Tunas,mackerels,andbillfishes(marlins,sailfishes,andswordfish)swimcontinuously.Feeding,courtship,reproduction,andeven"rest"arecarriedoutwhileinconstantmotion.Asaresult,practicallyeveryaspectofthebodyformandfunctionoftheseswimming"machines"isadaptedtoenhancetheirabilitytoswim.Manyoftheadaptationsofthesefishesservetoreducewaterresistance(drag).Interestinglyenough,severalofthesehydrodynamicadaptationsresemblefeaturesdesignedtoimprovetheaerodynamicsofhigh-speedaircraft.Thoughhumanengineersarenewtothegame,tunasandtheirrelativesevolvedtheir"high-tech"designslongago.Tunas,mackerels,andbillfisheshavemadestreamliningintoanartform.Theirbodiesaresleekandcompact.Thebodyshapesoftunas,infact,arenearlyidealfromanengineeringpointofview.Mostspecieslackscalesovermostofthebody,makingitsmoothandslippery.Theeyeslieflushwiththebodyanddonotprotrudeatall.Theyarealsocoveredwithaslick,transparentlidthatreducesdrag.Thefinsarestiff,smooth,andnarrow,qualitiesthatalsohelpcutdrag.Whennotinuse,thefinsaretuckedintospecialgroovesordepressionssothattheylieflushwiththebodyanddonotbreakupitssmoothcontours.Airplanesretracttheirlandinggearwhileinflightforthesamereason.Tunas,mackerels,andbillfisheshaveevenmoresophisticatedadaptationsthanthesetoimprovetheirhydrodynamics.Thelongbillofmarlins,sailfishes,andswordfishprobablyhelpsthemslipthroughthewater.Manysupersonicaircrafthaveasimilarneedleatthenose.Mosttunasandbillfisheshaveaseriesofkeelsandfinletsnearthetail.Althoughmostoftheirscaleshavebeenlost,tunasandmackerelsretainapatchofcoarsescalesneartheheadcalledthecorselet.Thekeels,finlets,andcorselethelpdirecttheflowofwateroverthebodysurfaceinsuchaswayastoreduceresistance(seethefigure).Again,supersonicjetshavesimilarfeatures.Becausetheyarealwaysswimming,tunassimplyhavetoopentheirmouthsandwaterisforcedinandovertheirgills.Accordingly,theyhavelostmostofthemusclesthatotherfishesuseto