Which extinguishing agent is NOT effective on Class B fires?

Water is not effective on Class B fires, which involve flammable liquids such as gasoline, oil, and grease. When water is used on these types of fires, it can actually spread the burning liquid, increasing the fire's spread and intensity. This is primarily because water is denser than many flammable liquids, causing it to sink and allowing the burning liquid to float on top, thereby exacerbating the fire situation.

In contrast, foam agents, dry chemical agents, and CO2 agents work by either smothering the fire or interrupting the chemical reaction of combustion. Foam agents create a barrier that prevents oxygen from reaching the fuel, while dry chemical agents can disrupt the combustion process, and CO2 agents displace oxygen around the fire. Hence, using water can be counterproductive and dangerous in a Class B fire scenario.