单项选择题
Due to （） action of pistons some relative motion between parts in contact in the coolant supply and return system must occur.
A.reciprocating B.rotating C.running D.removing
单项选择题 Doors are generally formed in the （） of the both in-line and Vee engines to give maintenance access to the crankshaft main and connecting rod large end bearing.
单项选择题 Dirty oil must be （） before being used again.
单项选择题 Dirt in the fuel oil system of a diesel engine can cause （）.
