单项选择题
Dirty oil must be （） before being used again.
A.mixed with clean oil B.purified C.heated D.strained
单项选择题 Dirt in the fuel oil system of a diesel engine can cause （）.
单项选择题 Diesel oil may be used to cool （） of a diesel engine.
单项选择题 Diesel engines are started by supplying （） into the cylinders in the appropriate sequence for the required direction.
