In a closed area, gases with vapor density of less than one will:

• A. rise and not concentrate near the ceiling.
• B. rise and concentrate near the ceiling.
• C. settle and not concentrate at the floor.
• D. settle and concentrate at the floor.

Answer: (B)

When a gas has a vapor density less than 1, it is lighter than air. In a closed area, this lighter-than-air gas will rise due to buoyancy and accumulate near the ceiling rather than the floor. It tends to form a layer at the top, especially if the space isn’t well mixed by ventilation. Gases heavier than air would do the opposite, settling toward the floor, which is why options describing a floor concentration or floor settling don’t fit. Therefore, the correct idea is that the gas rises and concentrates near the ceiling.