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What is the cause of "Spontaneous combustion" in materials like oily rags?

  1. External heating sources

  2. Water exposure

  3. Mechanical pressure

  4. Internal oxidation process

The correct answer is: Internal oxidation process

"Spontaneous combustion" occurs when a material bursts into flames without an external ignition source. This can happen when the material contains a high amount of volatile compounds, such as oils or solvents, which can chemically react with oxygen in the air and generate heat. This heat buildup can eventually reach a point where it reaches a self-sustaining combustion reaction and catches fire. Therefore, the correct cause of "spontaneous combustion" in materials like oily rags is an internal oxidation process. The other options, external heating sources, water exposure, and mechanical pressure, may contribute to the likelihood of a fire starting but they are not the primary cause.