单项选择题
A propeller, in order to operate efficiently, must rotate at a relatively （） speed.
A.high B.low C.fast D.lowly
单项选择题 Why a gearbox is needed in a ship driven by a medium-speed diesel engine?（）
单项选择题 Which one is not true?（）
单项选择题 The displacement of a ship is to the total weight, all told, of the relevant loaded ship（）
