Working alone at their respective constant rates, machine A and machine B can fill a certain order in 3 hours and 6 hours, respectively. If the two machines work simultaneously at their respective constant rates, how many hours does it take the two machines to fill 1/2 of that order?
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A\frac{1}{2}
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B\frac{3}{4}
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C1
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D\frac{11}{4}
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E\frac{11}{2}
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正确答案: C