單項選擇題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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2.單項選擇題Most marine biological sewage treatment plants consist of all the following statements except （）.

A.aeration tanks
B.settling tanks
C.disinfection agent
D.anaerobic bacteria

4.單項選擇題A check valve is another name for （）.

A.a test valve
B.an inspection valve
C.a stop valve
D.a non-return valve

5.單項選擇題A butterfly valve consists basically of a disc pivoted across the bore of a ring body having the same radial dimensions as the pipe in which it is fitted.According to the above sentence, which of the following statements is not true?（）

A.The diameter of a disc is the same as the outer diameter of the pipe
B.We can infer that the valve is opened or closed by rotating the spindle
C.A butterfly valve comprises a disc and a ring body
D.The profile of the butterfly valve is a ring body