单项选择题
A.（1）and（2）
B.（1），（2）and（3）
C.（1）and（3）
D.（2）and（3）
单项选择题 Attempts are made to eliminate axial thrust in centrifugal pump by （）.
单项选择题 The amount of fluid delivered by a gear pump can be regulated by （）.
单项选择题 Which of the following pumps are self-priming, and can deal with liquid from a level below the pump（） (1) ejector (2) screw (3) centrifugal