单项选择题
A.（1）and（2）
B.（2）and（3）
C.（1）and（3）
D.（1），（2）and（3）
单项选择题 Which of the following pumps will build up a bursting pressure or stall their motors if the discharge valves are closed?（） (1) gear (2) centrifugal (3) screw
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 （）.