Machine A, operating alone at its constant rate, produces 500 feet of a particular fiber in 2 hours. Machine B, operating alone at its constant rate, produces 500 feet of the same fiber in 3 hours. Machine C, operating alone at its constant rate, produces 500 feet of the same fiber in 5 hours. How many hours will it take machines A, B, and C, operating together at their respective constant rates, to produce 1,000 feet of the fiber?
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A\frac{60}{31}
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B\frac{15}{7}
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C\frac{25}{11}
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D\frac{10}{3}
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E\frac{20}{3}
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正确答案: A