🔍 Key Concepts

• Upper Explosive Limit (UEL) vs Lower Explosive Limit (LEL) for flammable vapors • What "too rich" and "too lean" mean in terms of fuel–air mixture • Fire triangle: fuel, oxygen, and ignition source

💭 Think About

• Think about what happens when there is more vapor than the upper explosive limit allows—does that make it easier or harder to burn? • Relate UEL and LEL to real-life examples, like adjusting the fuel/air mix on a gas stove or an engine—what happens if there is way too much fuel? • Which answer choice clearly describes the condition of the fuel–air mixture when it is beyond the range where combustion can be sustained?

✅ Before You Answer

• Verify that explosive/flammable range is always between the LEL and UEL, not above or below them. • Check which answer choices describe mixtures outside the flammable range rather than inside it. • Confirm for yourself whether "perfect for combustion" refers to being within the flammable range or beyond its limits.