题目内容：
根据以下材料，回答题Conventionalwisdomhaslongheldthatmammalsstayedmillionsofyearsonearth.Aslongasdinosaursroamedthelands,ourdistantancestorsnevergottobemuchmorethanchicken-heartedcreaturesthatsneakedoutatnighttograbbitsofplantswhentheterriblelizardswereasleep.
Onlywhentheywerewipedoutdidmammalsbegintoearnalittleevolutionaryrespect.
ButthatpicturechangeddramaticallywiththeannouncementinNatureoftwoimpressivefos-sils.One,ofabrand-newspeciesnamedR.giganticus,brokeapartthenotionthatmostdinosauragemammalswereneverlargerthansquirrels.Theanimalhadthedimensionsofamidsizedog-byfarthebiggestdinosaur-agemammaleverfound.Andthesecond,anewspecimenofapreviouslydiscoveredspeciescalledR.robustus,refutedthenotionthatitwasalwaysthemammalsthatgoteaten.Insidetheskeleton
wheretheanimal'sstomachwouldhavebeenarethefossilizedremainsofababydinosaur."Thisdiscoverywasthechanceofalifetime,"saysJinMeng,scientistandco-authorofthepaper.
Indeed,Mengdidn'texpecttofindthingslikethisatall.ThesmallerskeletonwasdiscoveredabouttwoyearsagobyvillagersinChina'sLiaoningprovince,siteofsomeoftherichestfossilbedsin
theworld.Theybroughtittotheattentionofscientists,whotookittoaninstituteforex-amination."Wedidn'tseethestomachcontentsatfirst,"saysMeng.
Aftertheydid,however,itdidn'ttakethemlongtorealizetheyhadstruckscientificgold.Oncloserexamination,thescientistsdeterminedthattheremainswerethoseofajuveniledinosaur.Someof
thearmandlegboneswerestillattachedtoeachother,suggestingthatR.robustusdidn'tchewitsfoodthoroughlybutwolfeditdowninlargechunks.
Takentogether,thefindsoverturnthealreadyerodedideathatearlymammalsweretinyandtimid.NowpaleontologistscanstopCookinguptheoriestoexplainwhymammalsweresolittlethat
theyhadtobesmalltoavoidbeingfound,forexample,ortheycouldn'tgrowlargerbecausedinosaursalreadyoccupiedthoseecologicalspaces.
Butit'snowclearthatmammalsdidfillsomeofthespacesreservedforlargeranimals."It'squitepossible,"sayspaleontologistAnneWeil,"thattheycompetedwithdinosaursforthesameprey.
"Andbecausetheyatedinosaurs,theymayevenhavehadaninfluenceondinosaurevolu-tion.Whatsortofinfluence?"Wedon'tknow,"shesays."That'showitiswiththebestfinds.Theyleaveyouwithmorequestionsthananswers."
Mammalsinthedinosaurageusedtobedescribedas A．fierceanddangerous.
B．shrewdandswift.
C．doglikeandsneaky.
D．smallandcowardly.
参考答案：
答案解析：