When inspecting piston rings through the ports of a two－stroke／cycle diesel engine, black

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

请帮忙给出正确答案和分析，谢谢！