单项选择题
Camshafts are usually driven by timing gears or （）.
A.push rods B.chain drives C.rocker arms D.flywheels
单项选择题 A waste heat boiler is installed on some diesel propelled vessels to （）.
单项选择题 What is the speed of the crankshaft in a four-stroke/cycle engine when the camshaft is turning at 750 rpm?（）
单项选择题 An electric heater built into some smaller diesel engines is used to （）.
