单项选择题 In order to keep the oil film on the cylinder liner during the running-in period, the piston rings are slightly rounded（）

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.