单项选择题
In four strokes engine, the cycle of events is completed in four strokes of （）.
A.the cylinder B.the crankshaft C.the piston D.the cross-head
单项选择题 In each cylinder cover there are two or three holes for （）
单项选择题 In each cylinder cover is a large central opening for mounting （）
单项选择题 In an ordinary four-stroke engine the exhaust valves open （）
