【正确答案】 D

【答案解析】
[听力原文] Questions <17>-<20>
When you take a walk in any of the cities in the West, you often see a lot of people walking dogs. It is still true that a dog is the most useful and faithful animal in the world, but the reasons why people keep a dog have 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 was also willing to obey his master. For example, when people used dogs for hunting, the dogs would not eat what was caught without permission. But in modern times people in the city need not protect themselves against attacks of animals. Why do they keep dogs, then? Some people keep dogs to protect themselves from robbery. 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.