单项选择题 The four-stroke engine is used for （） and sometimes for main propulsion with a gearbox to provide a propeller speed of between 90 to 120 rev/min.

单项选择题 The four strokes are called suction, compression, expansion and exhaust, but only （） stroke provides the power to drive the ship.

单项选择题 The fine filters will be arranged （） one can be cleaned while the other is operating.