阅读下面短文，按照句子结构的语法性和上下文连贯的要求，从各题所给的A, B,C D项中选出最佳选项。

Dogs are people's good friends and of great help to people. Dogs are very popular 1they are nice to people. They can help people 2. How do dogs help people? Some people can't see 3and they can't go out by 4very well. Dogs can lead these kinds of people5where they want to go. Some people are deaf. Dogs can let them 6when a phone rings or somebody calls. There are many kinds of dogs 7the world. Some people like to 8dogs for pets. Dogs make them 9and they can have fun with their dogs. But some dogs, 10sheep dogs, should not be pets because they working dogs. They can help people carry heavy things.