单选题The diameter of a piston is usually less at the crown than at the skirt, in order to （）.Afacilitate the installation of piston ringsBallow for the expansion of the piston during operationCprevent crankcase vapors from entering the combustion chamberDreduce wearing of the upper cylinder liner
单选题
The diameter of a piston is usually less at the crown than at the skirt, in order to （）.
A
facilitate the installation of piston rings
B
allow for the expansion of the piston during operation
C
prevent crankcase vapors from entering the combustion chamber
D
reduce wearing of the upper cylinder liner
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