Low-expansion foam has an expansion ratio of less than _____:1.

Low-expansion foam is characterized by an expansion ratio of less than 20:1. This means that the volume of foam generated is not more than 20 times greater than the volume of the liquid concentrate used to create it. This type of foam is primarily utilized for applications involving hydrocarbons and polar solvents, as it provides effective coverage and control over the fire.

The lower expansion ratio is significant because it allows for a denser foam that can penetrate through burning fuels and suppress flames more effectively. In fire suppression applications, such as involving flammable liquids, the properties of low-expansion foam help create a barrier between the fuel and the air, reducing the availability of oxygen and thereby controlling the fire.

The other options reflect higher expansion ratios that pertain to different foam types, such as medium and high-expansion foams, which serve different purposes and scenarios in fire suppression. Understanding the specific attributes of low-expansion foam is essential for effectively applying it in appropriate situations.