单选题The exhaust valve opens before bottom dead center in a four stroke engine to（） Ⅰ.allow for blow down Ⅱ.Reduce pumping lossesAⅠ onlyBⅡ onlyCboth Ⅰ and ⅡDneither Ⅰ nor Ⅱ
单选题
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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