单项选择题
A.butterfly valve
B.globe valve
C.gate valve
D.non-return valve
单项选择题 Adequate attention must be given to the water cooling system of the air compressors, for （） can cause condensation on the cylinder walls.
单项选择题 Bad valves operation of the reciprocating air compressors can be detected by observation of （）.
单项选择题 By far the greatest item of maintenance of the reciprocating air compressors concerns （）.