【单选题】

This is the minimum concentration of flammable vapors in air below which propagation of flame does not occur on contact with a source of ignition.

A、This is the maximum portion of flammable vapors in air. If the portion is bigger, when an ignition source touched the gas, it would cause the fire to be diffuseD.
B、This is the minimum volume of flammable vapors. If the volume is lower, when the gas contacted with an ignition source, the fire would not come out.
C、This is the minimum density of flammable vapors in air. If the density was lower, when an ignition source touched the gas, it would not cause the spread of flamE.