Class A foam acts as a surfactant, which means that it:

• A. Increases the viscosity of water.
• B. Eliminates fuel vapor formation.
• C. Lowers the surface tension of water, allowing better penetration into the fuel.
• D. Increases the density of water.

Answer: (C)

The key idea is how surfactants work. A Class A foam solution contains surface-active agents that lower the surface tension of water. When water’s surface tension is reduced, the water spreads and wets surfaces more easily, so it can penetrate into the fuel layer and into porous fuels rather than bead up on top. This improved wetting helps the foam blanket cling to the surface, absorbs heat more effectively, and provides better cooling and knockdown of the fire.

That’s why the correct statement is that it lowers the surface tension of water, enabling better penetration into the fuel. The other options aren’t the primary effect of a surfactant in Class A foam: it doesn’t principally increase water viscosity or density, and it doesn’t “eliminate” fuel vapor formation on its own.