单项选择题
For any piston ring to operate smoothly without scuffing ,the ring must be （）.
A.of a material harder than the cylinder liner B.properly lubricated C.prevented from compressing D.prevented from rotating during engine operation
单项选择题 Shaker, circulation and spray are the three general methods used in（）
单项选择题 Wristpin bearings are difficult to lubricate because of their oscillating motion and（）
单项选择题 Internal combustion engine crankcase vent outlets must be equipped with （）
