单项选择题
Valve overlap is the period during which （）.
A.the inlet valve is open B.the exhaust valve is open C.both valves are closed D.both valves are open
单项选择题 Valve gear knocks are more easily located while the engine （）.
单项选择题 VIT means （）
单项选择题 Under manoeuvring condition the main engine may run on （）.
