In the context of gas layer behavior during growth, what does the presence of isolated flames imply?

• A. Hot and cool gases are mixing.
• B. The oxygen concentration is low.
• C. Portions of the gas layer are within flammable range.
• D. Ignition is impossible.

Answer: (C)

Isolated flames show that the gas layer is not uniform and contains pockets where the fuel–air mixture falls within the flammable range. In those pockets, the concentration of fuel and oxygen is just right (between the lower and upper flammable limits), so an ignition source can ignite the gas and maintain a flame locally. Other areas of the layer may be too lean or too rich, or diluted, so they won’t ignite. This isn’t about the overall oxygen level being low or about ignition being impossible everywhere; it’s about specific spots having the right mixture to burn.