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（）.<br>A:pressure builds up in the nozzle which causes a surge of water<br>B:the only opening in a passageway is the one from which the nozzle is being advanced<br>C:pressure in the fire hose drops below 100 psi<br>D:a bulkhead collapses due to heat and pressure

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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