138. If heavy smoke is observed to be issuing from a partially open door of a paint locker having a CO2 fire extinguishing system, the FIRST firefighting attack should be to

A. Leave the door and vents open, but release the CO2 flooding system
B. Open the door completely to evaluate the extent of the fire
C. Enter and use a portable extinguisher
D. Secure the door and vents, then manually release the CO2