33. When an oil fire has been extinguished, the surface of the oil should be kept covered with foam to prevent __________.

Foam blankets prevent reignition by isolating flammable oil vapors from oxygen. Maintaining this barrier is crucial after extinguishing an oil fire, as hot oil continues to release vapors that can ignite if exposed to air. The foam's primary function is to suppress vapor release and prevent reflash, not to address toxic fumes, subsurface combustion, or boiling.