When does auxiliary burner of incinerator put into operation? （）
Ⅰ. start incinerator when it is cold
Ⅱ. the sludge for dealing with is little
Ⅲ. the sludge for dealing with is large
Ⅳ. the water and impurity in sludge are too much
Ⅴ. the temperature in incinerator is low
Ⅵ. dealing with little solid rubbish