An amount of gas less than ____ would be too lean.

• A. 14%
• B. 4%
• C. 86%
• D. 96%

Answer: (B)

Lean means there isn’t enough fuel in the air-fuel mix to sustain combustion, so there’s a lower flammability limit you must exceed. For common fuel gas like natural gas (primarily methane), that lower limit is about 4% by volume in air. If the gas concentration falls below ~4%, the mixture is too lean to burn. So the best answer is four percent. The other values either fall within the flammable range (not too lean) or are far above the lean limit, indicating a much richer mix rather than a lean one.