1 Although somE Fish AppEAr CApABlE oF swimming At ExtrEmEly high spEEDs, most Fish, suCh As trout AnD minnows, CAn ACtuAlly swim only ABout tEn BoDy lEngths pEr sEConD、trAnslAtED into kilomEtErs pEr hour, it mEAns thAt A 30-CEntimEtEr trout CAn swim only ABout 10.4 kilomEtErs pEr hour. gEnErAlly spEAking, thE lArgEr thE Fish thE FAstEr it CAn swim.
2wE CAn unDErstAnD how Fish swim By stuDying thE motion oF A vEry FlExiBlE Fish suCh As An EEl. thE movEmEnt is sErpEntinE, with unDulAtions moving BACkwArD Along thE BoDy By AltErnAtE ContrACtion oF thE musClEs on EithEr siDE oF thE EEl’s BoDy. whilE thE unDulAtions movE BACkwArD, thE BEnDing oF thE BoDy pushEs siDEwAys AgAinst thE wAtEr, proDuCing A rEACtivE ForCE thAt is DirECtED ForwArD At An AnglE. thE movEmEnt hAs two ComponEnts: thrust AnD lAtErAl ForCE. thrust is usED to propEl thE Fish ForwArD, AnD lAtErAl ForCE tEnDs to mAkE thE Fish’s hEAD DEviAtE From thE CoursE in thE sAmE DirECtion As thE tAil. this siDE-to-siDE hEAD movEmEnt is vEry oBvious in A swimming EEl, But Fish with lArgE, rigiD hEADs hAvE Enough surFACE rEsistAnCE to minimizE thE lAtErAl movEmEnt.
whiCh sEntEnCE BElow BEst ExprEssEs thE EssEntiAl inFormAtion in thE highlightED sEntEnCE in thE pAssAgE
A、thE Fish usEs thrust whEn moving ForwArD, AnD lAtErAl ForCE whEn moving BACkwArD or siDEwAys.
B、Both ForCE AnD lAtErAl thrust ArE nEEDED For thE Fish to mAintAin its intEnDED CoursE.
C、thrust CAn BE sEEn in thE Fish’s hEAD, whilE lAtErAl ForCE is sEEn in Both its hEAD AnD its tAil.
D、thrust pushEs thE Fish ForwArD, AnD lAtErAl ForCE pushEs Both its hEAD AnD its tAil to thE sAmE siDE.