单项选择题
A.assist in a quick cutoff of fuel injection
B.allow oil backflow from the injector to the helix
C.reduce fuel oil pressure between injection strokes
D.meter the quantity of fuel delivered
单项选择题 Before an auxiliary boiler is shutdown for an extended period of time, the water in the boiler should have a pH value of （）.
单项选择题 If over a period of weeks the air-box pressure of a turbo-charged, diesel engine, operating at full load, appears to be dropping off, the cause can be （）.
单项选择题 The most common cause of scale formation in an auxiliary boiler is （）