单项选择题
In general, （） is suitable for delivering small quantities at high pressure.
A.reciprocating pump B.gear pump C.centrifugal pump D.vane pump
单项选择题 In four strokes engine, the cycle of events is completed in four strokes of （）.
单项选择题 In each cylinder cover there are two or three holes for （）
单项选择题 In each cylinder cover is a large central opening for mounting （）
