For one or two types of engine, the piston and rod can be withdrawn downwards from the cylinder into the crankcase and then out through the （）.

For an ordinary modem marine four-stroke engine the inl...

单项选择题For an ordinary modem marine four-stroke engine the inlet valves close （）.

For an ordinary four-stroke engine the inlet valves ope...

单项选择题For an ordinary four-stroke engine the inlet valves open （）

For an ordinary four-stroke engine the exhaust valves o...

单项选择题For an ordinary four-stroke engine the exhaust valves open （）.