题目内容：
根据以下材料，回答题Humansliketoregardthemselvesasexceptional.Manyphilosophersbelievehumansaretheonly21whichunderstandsthatothershavetheirownpersonalthoughts.Thatunderstandingisknowninthe22ashavinga"theoryofmind,"anditisconsideredthe23tosuchcherishedhuman24assympathyanddeception.
Biologistshavelearnedtotreatsuch25withcaution.BerndHeinrichandThomasBugnyardescribeanexperimenttheyhavecardedout26ravens.
27togazeisreckonedtobeagood28ofthedevelopmentoftheoryofmindinhumanchildren.29about18months,mostchildren.areabletofollowthegazeofanotherperson,and30thingsaboutthegazerfromit.Failureto31thistrickisanearlysymptomofautism,asyndromewhosemainunderlyingfeatureisa(n)32tounderstandthatotherpeoplehave33,too.
to34whetherravenscouldfollowgaze，DrHeinrichusedSlXsix—month—oldhand—rearedravens.Thebirdswereset，oneatatime，onaperchononesideofaroomdividedbyabarrier.Anexperimenterinfrontofthebarrier35hisheadandeyesinaparticular36andgazedfor30secondsbeforelooking37.DrHeinrichfoundthatallthebirdswereabletofollowthegazeoftheexperimenters，even38thebarrier.Inthe39case，thecuriousbirdseitherjumpeddownfromtheperchandwalkedaroundthebarriertohavea40orleaptontopofitandpeeredover.
A．category
B．group
C．race
D．species
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