What is a common characteristic of flammable liquids in a Class B fire?

Flammable liquids are known for their ability to easily vaporize, which significantly contributes to their ignition potential. When these liquids are exposed to an open flame or a spark, the vapor can catch fire and lead to a Class B fire, which involves flammable liquids and gases. This characteristic is crucial for understanding how to effectively combat such fires, as it emphasizes the need for appropriate fire suppression methods that can handle the flammable vapor.

In contrast, the other options do not accurately represent the properties of flammable liquids. For example, flammable liquids do require oxygen to sustain combustion, making the second option incorrect. Additionally, while some flammable vapors may be lighter than air, this is not universally true, and thus, the claim about being lighter than air does not define the class as a whole. Lastly, the notion that they produce smoke that cools a fire contradicts the behavior of fire: smoke does not cool; it is often a product of incomplete combustion and can indicate the presence of heated gases that may contribute to the fire rather than helping to extinguish it.