题目内容：
根据以下材料，回答题Thedoghasalwaysbeenconsideredman'sbestfriend.Alwaysnotedforbeingparticularly
faithfulinwatchingoverchildren,healsohashisplacebythefireside,inthecowpasture,onthesheeprange(放牧区),andbesidethehunterinforest.Heiseasytotrain,workshard,andoftenperformsastonishingfeats.Andinthefrozenpolarregionshewasoncetheprincipalmotivepower,beforebeinglatelydisplacedbytheplaneandhelicopter.
Becausehehowlsorwhinesinthepresenceofimpendingdeath,thedogwasoncethoughttohavesupematuralpowersandbelievedtobecapableofseeinggodsandghostsinvisibletomen.Actual.ly,thebasisforthesebeliefsliesinthehound'ssensibilitytopeople'sfeelingsandhissuperiorhearingabilityandsenseofsmell,whichenablehimtodetectsignshiddenfromhumanobservation..Hisrecordofsavinglivesisoutstanding,forheoftengiveswamingoffireandotherdangersnotnoticedbyhismaster.
Thedog'smajorcontribution,however,hasbeentomedicalresearch.Bothhisdietandhis
structurearecomparabletothoseofthehumanbeing,andsohehasbeenthesubjectofcountless
demonstrationsandexperiments.Open-heartsurgeryhasbeenmadepossiblelargelybecauseofthedog.Buthissacrificehasrepaidhisownspeciesaswellbysafeguardingitfromrabies(狂犬病),distemper,andotherdiseases.
Thedoghasalwaysbeennotedfor. A．protectingchildren
B．assistingshepherds
C．helpinghunters
D．herdingcattle
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答案解析：