单项选择题
A.steam demand is high
B.salinity is high
C.safety valve simmers
D.burner flame is extinguished
单项选择题 Flame failure in an automatically fired auxiliary boiler can from a/an （）
单项选择题 In the event of a flame failure in an auxiliary water-tube boiler, you must（）
单项选择题 The purpose of a temperature sensing device installed in the stack of a small automatically fired auxiliary steam boiler is to secure the oil burner（）