单项选择题
In （） pump a closed discharge valve will not cause excessive pressure to build up.
A.gear B.vane C.screw D.centrifugal
单项选择题 Centrifugal pumps are non-positive and a fairly （） unit.
单项选择题 Gear pumps are （） type because they discharge a near constant amount of liquid regardless of pressure.
单项选择题 （） are more suitable for large quantities at low pressure.
