单项选择题 The sealing function of the piston ring is brought about by the gas pressure above and behind the piston ring, which forces it （） against the bottom of the ring groove, and （） against the cylinder wall.

单项选择题 The scavenge port inspection should be carried out（）, to obtain the most reliable result with regard to the effectiveness and sufficiency of the cylinder lubrication and the combustion cycle (complete or incomplete).

单项选择题 Always ensure that the turning gear is（）, even at the quay, the wake from other ships may turn the propeller and thus the engine.