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
