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 __________.

Correct Answer: Option B — the only opening in a passageway is the one from which the nozzle is being advanced

Advancing a high-velocity fog stream down a confined passageway with no alternative escape openings traps expanding steam and heated gases behind the nozzle. This forces a dangerous thermal flashback known as blow back toward the firefighting team.