单项选择题
A.agent B.shipchandler C.surveyor D.engineer
They understand the （） of automatic sludge discharging ...
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
These piston rings are so badly worn that they must be ...
单项选择题These piston rings are so badly worn that they must be （）.
A.renewed B.welded C.machined D.chromium-plated
There should be a lubricating oil （） between the main b...
单项选择题There should be a lubricating oil （） between the main beatings and the crankshaft.
A.film B.cap C.lacquer D.shell
