单项选择题 For one or two types of engine,the piston and rod can be withdrawn downwards from the cylinder into the crankcase and then out through （）.

单项选择题 For one or two types of engine, the piston and rod can be withdrawn downwards from the cylinder into the crankcase and then out through the （）.

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