单选题The crankcases of many diesel engines are kept under a slight vacuum to （）Aimprove fuel economyBincrease the air charge velocityCreduce the risk of explosionDall of the above
单选题
The crankcases of many diesel engines are kept under a slight vacuum to （）
A
improve fuel economy
B
increase the air charge velocity
C
reduce the risk of explosion
D
all of the above
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