Which statement about fuel vapors is high-yield?

• A. They are invisible, heavier than air, can travel with wind, and settle in low-lying areas
• B. They are visible and settle in high areas
• C. They are liquid at room temperature
• D. They are non-flammable

Answer: (A)

Fuel vapors behave as gases and are invisible to the eye. For hydrocarbon fuels, the vapor is typically heavier than air, so it tends to hug the ground and collect in low-lying areas, pits, drains, and depressions. They can spread with wind along the ground, not rise up and away. This combination of being invisible, ground-hugging, and transportable by air currents makes them especially dangerous and a priority in firefighting practice because you must account for where they can accumulate and how they can be carried to ignition sources. That’s why this description—unseen, heavier than air, capable of moving with wind, and pooling in low areas—is the most useful and high-yield. The other statements don’t fit: vapors are not visible and don’t preferentially settle in high areas; vapors are gaseous, not liquid at room temperature; and fuel vapors are flammable, not non-flammable.