Which statement about the fire triangle is true?

• A. The chemical chain reaction is one of the four elements.
• B. Heat is not part of the triangle.
• C. Oxygen is not part of the triangle.
• D. A single element can sustain fire by itself.

Answer: (A)

The idea being tested is that fire is sustained by more than just heat, fuel, and oxygen. In the extended model sometimes called the fire tetrahedron, the chemical chain reactions inside the flame are treated as a fourth essential element. These chain reactions involve free-radical reactions (like H, O, OH) that propagate the combustion process, maintaining the flame and its high temperatures. If you can disrupt these radical reactions—by cooling, starving the fuel, or reducing the oxygen—the flame can no longer sustain itself. So describing the chemical chain reactions as one of the four elements is true in this broader framework. The other statements are not correct: heat and oxygen are components of the traditional trio, and a single element by itself cannot sustain combustion under normal conditions.