单项选择题
Fuel oil tanks must be fitted with （） that can be closed rapidly and remotely in the event of an emergency such as a fire.
A.stop valves
B.non-return valves
C.quick closing valves
D.relief valves
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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
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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
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Most marine biological sewage treatment plants consist of all the following statements except （）.
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C.disinfection agent
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