单项选择题
This used piston had been tested and approved as a spare part by the （） during last annual repair.
A.agent B.shipchandler C.surveyor D.engineer
单项选择题 They understand the （） of automatic sludge discharging of the purifierBut they are not good in practice.
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.
