单项选择题
If the capacity of a centrifugal pump decreases gradually over a long period of time, you should replace the （）.
A.packing gland B.mechanical seals C.lantern rings D.wearing rings
单项选择题 Distilled water from an evaporator may be discharged into a potable water tank （）.
单项选择题 If a centrifugal pump vibrates and is noisy when operating, the cause could be （）.
单项选择题 Which of the devices listed is common to all types of pumps?（）
