单项选择题
A.suction
B.compression
C.expansion
D.exhaust
单项选择题 The fine filters will be arranged （） one can be cleaned while the other is operating.
单项选择题 The film of oil between the rings and the liner is not only essential for the lubrication but also （） achieving a gas seal.
单项选择题 The film of oil between the rings and the liner is not only essential for the lubrication but also plays an important part in achieving （）.