单选题When inspecting piston rings through the ports of a two-stroke/cycle diesel engine, black areas on the sealing surfaces are the result of （）Ainsufficient lubricationBimproper piston coolingCblow-byDoverload operation
单选题
When inspecting piston rings through the ports of a two-stroke/cycle diesel engine, black areas on the sealing surfaces are the result of （）
A
insufficient lubrication
B
improper piston cooling
C
blow-by
D
overload operation
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