It takes 6 beavers 10 hours to build a certain dam, working at a uniform rate. If six beavers start to build the same dam at noon, and one beaver per hour is added beginning at 6:00 PM, at what time will the dam be complete?
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A7:40 PM
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B8:00 PM
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C8:20 PM
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D8:40 PM
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E9:00 PM
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正确答案: E