单项选择题
A.caustic embrittlement of the boiler metal
B.acidic corrosion of the boiler metal
C.hard scale deposits on the boiler tubes
D.etching of the heat exchange surfaces
单项选择题 The purpose of the delivery check valve used in a diesel fuel injection jerk pump is to （）.
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 （）.