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Mr. Black had two cats. One was very big and the other was quite small. He liked them very much. One day, Mr. Green, a friend of his, came to see him. To his great surprise, he saw two holes （洞）in the door, a large hole and a small hole. He said, “My dear friend, why are there two holes in your door?” “Let the cats in and out, ” Mr. Black answered.“ But why are there two holes? Isn't one hole enough?” asked his friend. “How can the big cat go through the small hole? He said.