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

Flammability limits define the range where combustion occurs. The Upper Explosive Limit (UEL) represents the maximum concentration of fuel vapor in air that can support combustion. Above this threshold, the mixture is too fuel-rich to sustain a flame, effectively rendering it non-explosive due to insufficient oxygen for the reaction.