单项选择题 Engine operating conditions may be indicated by the color of the exhaust smoke.Black smoke could indicate（）

单项选择题 A dark exhaust from a running diesel engine can be caused by （）

单项选择题 What color exhaust will be exhibited when a slow speed two-stroke/cycle main propulsion diesel engine, designed to operate on light and heavy fuel oil, is operated on insufficiently preheated heavy fuel oil?（）