单项选择题
They understand the （） of automatic sludge discharging of the purifierBut they are not good in practice.
A.principle B.principal C.cause D.case
单项选择题 These piston rings are so badly worn that they must be （）.
单项选择题 There should be a lubricating oil （） between the main beatings and the crankshaft.
单项选择题 There is an injection pump for each cylinder, which is usually operated once every cycle by （） on the camshaft.
