The upper explosive limit (UEL) of a mixture of flammable vapors and air is defined as __________.

Correct Answer: Option B — that concentration above which the mixture is too rich to burn

Flammable mixtures possess a specific range of concentrations in air. The UEL represents the maximum concentration of vapor; beyond this point, the mixture contains insufficient oxygen to support combustion, rendering it too rich to ignite. This principle is fundamental to managing fuel-air mixtures in engine room spaces.