单项选择题

The dog has been of most value to man m the development of （）

A．a treatment for rabies
B．open-heart surgery
C．a cure for distemper
D．all of the above

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单项选择题
The article does not say whether the scientists' experiments with dogs have（）

A．benefited animals other than dogs
B．served man
C．helped other dogs
D．contributed to medical knowledge
单项选择题
In the polar regions, the dog mainly （）

A．carried supplies
B．provided companionship
C．tracked prey
D．herded caribou(驯鹿)
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