Which category includes materials that burn due to friction or metal powders that can ignite?

Which category includes materials that burn due to friction or metal powders that can ignite?

• A. Ignition only by electric spark.
• B. Cryogenic temperatures.
• C. Friction, rapid burning (over 2.2 mm per second), or ignition of metal powders.
• D. Ignition only when in contact with water.

Answer: (C)

The idea being tested is how certain flammable solids ignite. Some materials can catch fire from friction or from the presence of metal powders, which can ignite because powders have a large surface area that oxidizes rapidly. The best category explicitly describes these modes: friction, rapid burning (a defined rate), or ignition of metal powders. It directly covers both friction-induced ignition and metal-powder ignition, which is exactly what the question asks about. The other options describe ignition scenarios that don’t match this specific group: ignition only by electric spark, or ignition related to cryogenic temperatures, or ignition only when in contact with water. Those describe different hazards and do not capture the friction- or metal-powder ignition scenario.

The idea being tested is how certain flammable solids ignite. Some materials can catch fire from friction or from the presence of metal powders, which can ignite because powders have a large surface area that oxidizes rapidly. The best category explicitly describes these modes: friction, rapid burning (a defined rate), or ignition of metal powders. It directly covers both friction-induced ignition and metal-powder ignition, which is exactly what the question asks about.

The other options describe ignition scenarios that don’t match this specific group: ignition only by electric spark, or ignition related to cryogenic temperatures, or ignition only when in contact with water. Those describe different hazards and do not capture the friction- or metal-powder ignition scenario.