All the rings are slightly rounded on their external top and bottom edges to keep the oil

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All the rings are slightly rounded on their external top and bottom edges to keep the oil film on the cylinder liner during _____ for new piston rings.

A.the running-in period

B.the running process

C.gas exchange process

D.the boiler firing

请帮忙给出正确答案和分析,谢谢!

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