The exhaust valve opens before bottom dead center in a four stroke engine to______Iallow f

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The exhaust valve opens before bottom dead center in a four stroke engine to______Iallow for blow down IIReduce pumping losses

A.I only

B.II only

C.both I and II

D.neither I nor II

请帮忙给出正确答案和分析,谢谢!

热搜:a是b的充分条件,b是a的必要条件吗