單項(xiàng)選擇題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


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1.單項(xiàng)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

3.單項(xiàng)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

4.單項(xiàng)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

5.單項(xiàng)選擇題Unlike the globe valve, gate valves give full bore flow without change of direction, such a valve is not suitable to partially open operationAccording to the above sentence, which of the following statements is not true?（）

A.The globe valve may be suitable for partially open operation
B.The globe valve will give full bore flow with change of direction
C.The gate valve gives full bore flow with change of direction
D.The gate valve is suitable for full bore operation only