The dog has always been considered man′s best friend.Always note

The dog has always been considered man′s best friend.Always noted for being particularly?faithful in watching over children,he also has his place by the fireside,in the cow pasture,on the?sheep range(放牧区),and beside the hunter in forest.He is easy to train,works hard,and often?performs astonishing feats.And in the frozen polar regions he was once the principal motive power,before being lately displaced by the plane and helicopter. Because he howls or whines in the presence of impending death,the dog was once thought to?have supernatural powers and believed to be capable of seeing gods and ghosts invisible to men.Actually,the.basis for these beliefs lies in the hound′s sensibility to people′s feelings and his superior?hearing ability and sense of smell,which enable him to detect signs hidden from human observation.His record of saving lives is outstanding,for he often gives warning of fire and other dangers not noticed by his master. The dog′s major contribution,however,has been to medical research.Both his diet and his?structure are comparable to those of the human being,and so he has been the subject Of countless?demonstrations and experiments.Open-heart surgery has been made possible largely because of the?dog.But his sacrifice has repaid his own species as well by safeguarding it from rabies(狂犬病),distemper,and other diseases. In the polar regions,the dog mainly__________.
A、carried supplies
B、provided companionship
C、tracked prey
D、herded caribou（驯鹿）
【正确答案】：A
【题目解析】：由文段第一段最后一句“And in the frozen polar regions he was once the principal motive power, before being lately displaced by the plane and helicopter.”可知，狗曾是极地的主要动力，飞机和直升机出现后才取代了它。A 项“carried supplies”“搬运物资”符合狗作为动力的作用，故 A 选项正确；B 项“provided companionship”“提供陪伴”、C 项“tracked prey”“追踪猎物”、D 项“herded caribou（驯鹿）”“放牧驯鹿”均不符合狗在极地的主要贡献。