单项选择题
A.on both the external and internal top edges
B.on both the external top and bottom edges
C.on both the external and internal bottom edges
D.on all the four edges
单项选择题 In some type of two-stroke engines, besides some small holes for（）, starting valve, safety valve, indicator cock, and central opening in each cylinder cover for mounting the（）
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（）.