单项选择题 However, where immediate action is necessary to ensure safety of the ship, its machinery and crew, it must be taken by the engineer in chargeWhat dose “it” mean in the sentence above?（）

单项选择题 There will usually be summaries or daily account tables for heavy oil, diesel oil, lubricating oil, and fresh water, which will be compiled（）

单项选择题 According to the principle of two stroke diesel engine, what is the number of its efficient work stroke per revolution?（）