【答案解析】[听力原文]
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. Later they came to realize that a dog was not only useful for protection / but also willing to obey his master. When people used dogs for hunting, the dogs would not eat what was caught without permission. Now people in the city need not protect themselves against attacks of animals. Some people keep dogs / to protect themselves from robbery or 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. The reason why people keep dogs has changed / from protection to friendship.