Passage1 Plantsandanimalsthathavebeenstudiedcarefullyseemtohavebuilt-inclocks.Thesebiologicalclocks,astheyarecalled,usuallyarenotquiteexactinmeasuringtime.However,theyworkprettywellbecausetheyare"reset"eachday,whenthesuncomesup. Dopigeonsusetheirbiologicalclockstohelpthemfinddirectionsfromthesun?Wecankeeppigeonsinaroomlitonlybylamps.Andwecanprogramthelightingtoproduceartificial"days",differentfromthedayoutside.Afterawhilewehaveshiftedtheirclocks.Nowwetakethemfarawayfromhomeandletthemgoonasunnyday.Mostofthegnstartoutasiftheyknowjustwhichwaytogo,butchooseawrongdirection.Theyhavepickedadirectionthatwouldbecorrectforthepositionofthesunandthetimeofdayaccordingtotheirshiftedclocks. Itisknownandexperimentedthathomingpigeonscantelldirectionsbythesun.Butwhathappenswhentheskyisdarklyovercastbycloudsandnoonecanseewherethesunis?Thenthepigeonsstillfindtheirwayhome.Thesalneexperimenthasbeenrepeatedmanytimesonsunnydaysandtheresultwasalwaysthesame.Butonveryovercastdaysclock-shiftedpigeonsarejustasgoodasnormalpigeonsinstartingoutintherightdirections.Soitseemsthatpigeonsalsohavesomeextrasenseofdirectiontousewhentheycannotseethesun. Naturally,peoplehavewonderedwhetherpigeonsmighthaveabuilt-incompass—somethingthatwouldtellthemaboutthedirectionsoftheearth'smagneticfield.Onewaytotestthatideawouldbetoseeifapigeon'ssenseofdirectioncanbefooledbyamagnetattachedtoitsback. Withastrongmagnetcloseby,acompasscannolongertelldirection. Totesttheidea,agroupoftenpigeonshadstronglittlemagnetbarsattachedtotheirbacks. Anothergroupcarriedbrassbarsinsteadwhichwerenotmagnetic.Inanumberofexperiments,bothgroupsweretakenawayfromhomeandletgo.Onsunnydaysnoneofthemagnet-pigeonswasfooled.Theywerejustasgoodasthebrass-pigeonsinstartingoutintherightdirectiontowardhome.Oncloudy,overcastdays,however,withnosunthebrass-pigeonschosetherightdirection,butthemagnet-pigeonswereintrouble.Theylaterstartedoutindifferentdirectionsandactedcompletelylost. Whatcanbeinferredfromthepassageaboutbiologicalclocks? 查看材料
A.They are used by all plants and animals to tell time. B.Pigeons' biological clocks are regulated every day with the sunrise. C.The built-in biological clocks cannot be changed. D.They gradually developed as pigeons grow mature.正确答案B
