单项选择题
Each piston stuffing box is provided with two sealing rings and three（）
A.packings B.piston rings C.distributor rings D.scraper rings
单项选择题 Each journal and pin should be examined for （） and grooving.
单项选择题 Each cylinder liner on the marine medium speed engines （）.
单项选择题 Each cylinder is closed at the top by a （）, made of a special heat-resistant steel.
