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
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A.bridges
B.recesses
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Each cylinder liner on the marine medium speed engines （）.
A.is usually cooled by sea water
B.can be replaced when worn out
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单项选择题
Each cylinder is closed at the top by a （）, made of a special heat-resistant steel.
A.cylinder liner
B.coupling flange
C.cylinder cover
D.piston crown
