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The major difference between the discharge and suction valves installed in most low pressure, reciprocating air compressors is that（）
A:one valve seats upwards, while the other seats downwards
B:the reed valves used on the discharge are made substantially thicker and heavier than the suction valves
C:the suction valve springs exert a greater tension than the discharge valve springs
D:the discharge valve springs exert a greater tension than the suction valve springs

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出自：海船船员证

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