单项选择题
The pressure difference before and after the lube oil filter indicates （）the filter should be cleaned.
A.when B.how C.where D.whether
单项选择题 A diesel engine which is rated for normal operation at a crankshaft speed of 800 RPM, is commonly classed as a （）.
单项选择题 The three basic parts of any eductor are the nozzle, the suction chamber, and the （）.
单项选择题 （）, the engine must be stopped immediately.
