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（）.
- Apressure builds up in the nozzle which causes a surge of water
- Bthe only opening in a passageway is the one from which the nozzle is being advanced
- Cpressure in the fire hose drops below 100 psi
- Da bulkhead collapses due to heat and pressure