单项选择题 In a two-stroke engine, there are always a series of openings known as （）, and in some circumstances, there are also openings known as （）, in the part of the cylinder liner.

单项选择题 If the engine is naturally aspirated, or is a small high-speed type with a centrifugal turbocharger, the period of valve overlap（）,ie when both valves are（）, will be and the exhaust valve will close some 10°top dead canter（）.

单项选择题 （）is the main reason that reduces the power advantage of a two-stroke engine over a same size four-stroke one to about（）