单项选择题
A more sophisticated pressure reducing valve incorporates a pilot valve which controls the fluid above the valve operating piston in response to downstream pressureThe bold and italic word “which” means （）.
A.a pressure reducing valve
B.a pilot valve
C.an operating piston
D.downstream pressure
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In its simplest form the pressure reducing valve is opened or closed by （） which is balanced by the downstream pressure acting on a diaphragm.
A.adjusting a nut
B.a pre-tensioned spring
C.any spring
D.upstream pressure -
单项选择题
Most marine biological sewage treatment plants consist of all the following statements except （）.
A.aeration tanks
B.settling tanks
C.disinfection agent
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单项选择题
To prevent accidents from occurring, special valve chests are used in such cases in which interchangeable blanks and connecting passages are incorporatedThe bold and italic word “which” means （）.
A.accidents
B.special valve chests
C.interchangeable blanks
D.connecting passages
