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 spring