11.Anewstudyonbirds'sleephasrevealedthat____________.A)half-brainsleepisfoundinawidevarietyofbirdsB)half-brainsleepischaracterizedbyslowbrainwavesC)birdscancontroltheirhalf-brainsleepconsciouslyD)birdsseldomsleepwiththewholeoftheirbrainatrest12.Accordingtothepassage,birdsoftenhalfsleepbecause______________.A)theyhavetowatchoutforpossibleattacksB)theirbrainhemispheretaketurnstorest跨段C)thetwohalvesoftheirbrainaredifferentlystructured没D)theyhavetoconstantlykeepaneyeontheircompanions反13.Theexampleofabirdsleepinginfrontofamirrorindicatesthat_____________.A)thephenomenonofbirdsdozinginpairsiswidespreadB)birdsprefertosleepinpairsforthesakeofsecurityC)evenanimaginedcompaniongivesthebirdasenseofsecurityD)asinglepetbirdenjoysseeingitsownreflectioninthemirror14.Whilesleeping,somewatermammalstendtokeephalfawakeinorderto__________.A)alertthemselvestotheapproachingenemyB)emergefromwaternowandthentobreatheC)besensitivetotheever-changingenvironmentD)avoidbeingsweptawaybyrapidcurrents15.By"justthetipoftheiceberg"(Line2,Para.8),Siegelsuggeststhat____________.A)half-brainsleephassomethingtodowithicyweatherB)themysteryofhalf-brainsleepisclosetobeingsolvedC)mostbirdslivingincoldregionstendtobehalfsleepersD)half-brainsleepisaphenomenonthatcouldexistamongotherspeciesBirdsthatareliterallyhalf-asleep-withonebrainhemispherealertandtheothersleeping-controlwhichsideofthebrainremainsawake,accordingtoanewstudyofsleepingducks.Earlierstudieshavedocumentedhalf-brainsleepinawiderangeofbirds.Thebrainhemispherestaketurnssinkingintothesleepstagecharacterizedbyslowbrainwaves.Theeyecontrolledbythesleepinghemispherekeepsshut,whilethewakefulhemisphere'seyestaysopenandalert.Birdsalsocansleepwithbothhemispheresrestingatonce.Decadesofstudiesofbirdflocksledresearcherstopredictextraalertnessinthemorevulnerable,end-of-the-rowsleepers,Sureenough,theendbirdstendedtowatchcarefullyonthesideawayfromtheircompanions.Ducksintheinnerspotsshowednopreferenceforgazedirection.Also,birdsdozing（打盹）attheendoftheliner...
