单项选择题
When using a high-velocity fog stream in a passageway，the possibility of a blow back must be guarded against. Blow back is most likely to occur when（）.
A.pressure builds up in the nozzle which causes a surge of water
B.the only opening in a passageway is the one from which the nozzle is being advanced
C.pressure in the fire hose drops below 100 psi
D.a bulkhead collapses due to heat and pressure
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When used to fight fire，carbon dioxide（）.
A.is effective if used promptly on an oil fire
B.has a greater cooling effect than water
C.is lighter than air
D.is harmless to cargo and crew -
单项选择题
When two fire hose teams are attacking a fire they should（）.
A.use different fire hose pressures
B.use fire hoses of different sizes
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A.First aid kit
B.Axe or cutting torch
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