单项选择题
If a centrifugal pump vibrates and is noisy when operating, the cause could be （）.
A.worn gland sealing shaft B.worn wearing rings C.a bent shaft D.reversed pump coupling
单项选择题 Which of the devices listed is common to all types of pumps?（）
单项选择题 A centrifugal pump may fail to deliver water when first started if the （）.
单项选择题 Which features of a centrifugal pump reduce the need for renewing worn impellers and pump casings?（）
