The flammable range is BEST described as:

• A. The percentage of gas or vapor concentration in the air
• B. The rate of gas expansion
• C. The danger rating of the material
• D. The viscosity of the vapor

Answer: (A)

The concept being tested is that the flammable range refers to the concentration of vapor in air that can ignite. It is described as a percent by volume of the vapor in air, bounded by the lower flammable limit and the upper flammable limit. Below the lower limit the mixture is too lean to ignite, above the upper limit it’s too rich to ignite, and within that window ignition is possible if an ignition source is present. The other statements describe how a gas behaves or its properties (rate of expansion, overall danger rating, viscosity) rather than the concentration range that can burn.