325. 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