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Which of the following statements is true concerning the overall efficiency of air compressors?（）
A:High pressure compressors are more efficient than low pressure compressors
B:Mechanical efficiency divided by compressor efficiency will equal the overall efficiency of the compressor unit
C:Two stage compressors are generally more efficient than single stage compressors
D:The volumetric efficiency is decreased in multistage compressors

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出自：海船船员证

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