After each reading of an oxygen indicator，the instrument should be purged with（）.

A.fuel supply
B.exhaust ventilation
C.supply and exhaust ventilation
D.mechanical and natural ventilation

A.over pressurization of the space being flooded
B.rupture of cylinder due to temperature increase
C.over pressurization of the CO₂ discharge piping
D.flooding of a space where personnel are present

A.contaminating food with extinguishing agent
B.spreading through the engineering space
C.causing loss of stability
D.a grease fire in the ventilation system