单项选择题
The young motorman as well as the others （）to operate the oil separator.
A.has learnt B.have learnt C.learn D.are learning
单项选择题 No sooner （） than they started to repair the sludge pump.
单项选择题 （）the moveable plate and floating ring inside the pump is very difficult.
单项选择题 The four-stroke cycle begins with the piston （）.
