What defines the flammable range?

The flammable range specifically refers to the range of concentrations of a vapor or gas in air that can ignite when an appropriate ignition source is present. This range is delineated by the upper and lower flammable limits (UFL and LFL).

When the concentration of a combustible material's vapor in air is below the lower flammable limit, there is not enough fuel present to support combustion, while if the concentration exceeds the upper flammable limit, there is too much fuel for the air to sustain combustion. Thus, the flammable range defines the precise parameters within which combustion can occur, explaining why this answer is the correct choice. Understanding this concept is critical for fire and explosion investigators as it helps them evaluate the potential for ignition in various scenarios.