单项选择题
A.smaller, higher
B.higher, smaller
C.smaller, smaller
D.higher, higher
单项选择题 In order to control thermal stresses, some types of modern engines use pistons with（）
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（）