The major difference between the discharge and suction valves installed in most low pressure, reciprocating air compressors is that（）
- Aone valve seats upwards, while the other seats downwards
- Bthe reed valves used on the discharge are made substantially thicker and heavier than the suction valves
- Cthe suction valve springs exert a greater tension than the discharge valve springs
- Dthe discharge valve springs exert a greater tension than the suction valve spring