单项选择题
A.scavenging action of the piston
B.piston type vacuum pump taking suction off a differential manometer
C.gland exhausting manometer
D.crankcase exhaust fan
单项选择题 Fins are installed on the generating tube surfaces in waste heat boilers to （）.
单项选择题 On a medium-speed main propulsion diesel engine, the crank-pin or crank journal bearings receive lubricating oil from （）.
单项选择题 The rate of cylinder lubricating oil metered to each cylinder of a large low-speed main propulsion diesel engine is （）