单项选择题

The dog has always been considered man’s best friend. He’s always noted for being particulcarly faithful in watching over children, he also has his place by the fireside, in the cow pasture, on the sheep range, and beside the hunter or the blind person. He is easy to train, works hard, and often performs astonishing feats. And in the frozen Polar Regions, he was once the principal source of motive power, before being largely replaced 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 people. Actually, the basis for these beliefs lies in the dog’s sensitivity 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 made to medical research. Both his diet and his structure are comparable to those of the human being, 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.

The article does not say whether the scientists’ experiments with dogs have（）

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