【正确答案】 B

【答案解析】[听力原文]17-20
When you take a walk in any of the cities in the West, you often see a lot of people walking with dogs. It is still true that the dog is the most useful and faithful animal in the world. But the reason why people keep a dog has changed. In the old days people used to train dogs to protect themselves against attacks by other beasts. And later they came to realize that a dog was not only useful for protection but willing to obey his master. For example, when people used dogs for hunting, the dogs wouldn't eat what was caught without permission. But now people in a city need not protect themselves against attacks from animals. Why do they keep dogs then? Some people keep dogs to protect themselves from thieves. But the most important reason is for companionship. For a child, a dog is his best friend when he has no friends to play with. For young couples, a dog is their child when they have no children. For old couples, a dog is also their child when their real children have grown up. So the main reason why people keep dogs has changed from protection to friendship.

独白第四句指出：In the old days... to protect themselves against attacks by other beasts，即以前的人训练狗是为了防止自己被野兽攻击，因此选[B]考生只要听清了与in the old days对应的protect一词，或者根据常识即可确定答案。