阅读下面短文，从每题所给的A、B、C、D四个选项中选出最佳选项。C
    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 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(练) dogs to protect themselves against attacks by other animals.
    But now people in this city need not protect themselves against attacks of animals. Why do they keep dogs, then?   
    Some people keep dogs to protect themselves from robber(抢) but the most important reason is for companionship. For a child, a dog is his best friend when he has no friend to play with. For old couples, a dog is also their child when their real child has grown up. So the main reason why people keep dogs has changed from protection to friendship.