Which of the following is not a common method to fight Class B fires?

Fighting Class B fires, which involve flammable liquids such as gasoline, oil, and grease, requires specific methods tailored to the unique properties of these materials. Water is not an effective method for extinguishing Class B fires because it can actually spread the fire by floating on top of the burning liquid and potentially causing an explosive flare-up. This can lead to greater danger and difficulty in controlling the blaze.

In contrast, foam, dry powder, and carbon dioxide are designed to smother the fire or interrupt the chemical reaction that is sustaining it. Foam is particularly effective because it forms a barrier that suppresses vapors and cools the burning liquid. Dry powder agents work by interrupting the combustion chemical process, while carbon dioxide displaces oxygen and reduces the fire's ability to continue burning. Therefore, water is the least appropriate method for fighting Class B fires, making it the correct answer in this context.