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

The Upper Explosive Limit (UEL) represents the maximum concentration of flammable vapor in air that can support combustion. Above this threshold, the mixture is too rich in fuel and lacks sufficient oxygen to sustain a flame, rendering the atmosphere non-explosive, though still potentially toxic or asphyxiating for personnel.