The four elements of a fire are oxygen, heat, ______, and ______ chain reaction.

The four elements of a fire are oxygen, heat, ______, and ______ chain reaction.

• A. FUEL; HEAT
• B. OXYGEN; HEAT
• C. CHEMICAL; FUEL
• D. FUEL; CHEMICAL

Answer: (D)

The fire tetrahedron shows four elements needed for a flame: heat, oxygen, fuel, and the chemical chain reactions that occur during combustion. The missing pieces are fuel and chemical, so the sentence reads: oxygen, heat, fuel, and chemical chain reaction. Fuel provides the material to burn, and the chemical chain reactions drive the flame chemistry that keeps combustion going. Without fuel, there’s nothing to burn; without the chemical chain reactions, the flame can’t sustain itself even if heat and oxygen are present. The other options would either repeat an existing element or place terms in a way that doesn’t describe the flame’s chemistry.

The fire tetrahedron shows four elements needed for a flame: heat, oxygen, fuel, and the chemical chain reactions that occur during combustion. The missing pieces are fuel and chemical, so the sentence reads: oxygen, heat, fuel, and chemical chain reaction. Fuel provides the material to burn, and the chemical chain reactions drive the flame chemistry that keeps combustion going. Without fuel, there’s nothing to burn; without the chemical chain reactions, the flame can’t sustain itself even if heat and oxygen are present. The other options would either repeat an existing element or place terms in a way that doesn’t describe the flame’s chemistry.