What key factor determines the extinguishing agent in a fire extinguisher?

The key factor that determines the extinguishing agent in a fire extinguisher is the class of fire it is designed to combat. Fire classification is based on the materials involved in the fire, such as solid combustible materials, flammable liquids, gases, and electrical equipment. Different types of fires require specific extinguishing agents to be effective and safe. For example, water is commonly used for Class A fires (ordinary combustibles), while Class B fires (flammable liquids) require agents like foam or dry chemical extinguishers that can effectively smother the fire and prevent re-ignition. Understanding the class of fire ensures that the correct extinguishing agent is used, maximizing effectiveness and safety during firefighting efforts.