单项选择题

In a medium speed diesel engine, a trunk type piston may be cooled by()
Ⅰ.oil circulation though passages in the piston crown
Ⅱ.heat transfer through piston rings and liner wall

A.Ⅰ only
B.Ⅱ only
C.either Ⅰ or Ⅱ
D.neither Ⅰ or Ⅱ