單項(xiàng)選擇題

In a diesel engine, exhaust valves open before the intake ports are uncovered to ()
Ⅰ.reduce pumping losses
Ⅱ.reduce back pressure

A.Ⅰonly
B.Ⅱonly
C.bothⅠand Ⅱ
D.neitherⅠnor Ⅱ

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