單項選擇題

The exhaust valve opens before bottom dead center in a four stroke engine to()
Ⅰ.allow for blow down
Ⅱ.Reduce pumping losses

A.Ⅰ only
B.Ⅱ only
C.both Ⅰ and Ⅱ
D.neither Ⅰ nor Ⅱ

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