单项选择题
For one or two types of engine, the piston and rod can be withdrawn downwards from the cylinder into the crankcase and then out through the （）.
A.cylinder casing B.cylinder bore C.crankcase door D.aperture in the cylinder block
单项选择题 For an ordinary modem marine four-stroke engine the inlet valves close （）.
单项选择题 For an ordinary four-stroke engine the inlet valves open （）
单项选择题 For an ordinary four-stroke engine the exhaust valves open （）.
