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Mr. Black has two _________. One is big and the other is small. He likes them very much. One day his good friend Mr. Green comes to _________ him. He finds there are two holes(洞) in the _________, a big hole and a small one. He says, “My dear friend, why are there two holes in your door?” “To let my dogs _________ and out,” Mr Black answers. “But one hole is OK.” says his friend. “How can the big dog go through(通过) the _________ hole?” Mr. Black asks.