问题补充：
When you take a walk in any of the cities in the West, you often see a lot of people walking dogs. 48is still true that a dog is the 49useful and faithful animal in the world, 50the reason why people keep a dog has changed. In the old days people used to train dogs to protect themselves against animal attacks. And later they came to realize that a dog was not 51useful for protection but willing to obey his master. But now people in the city need not protect themselves against attacks of animals. 52
do they keep dogs, then? Some people keep dogs to protect themselves from robbery. But the most important reason is 53companionship. A dog is his best friend for
54child when he has no friends to play with. For young couples, a dog is their child when they have 55children. For old couples, a dog is also their child when their real children have grown up.48. It 49. most 50. but 51. only
答案：
(答案→)48. It 49. most 50. but 51. only
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