Which of the following is NOT a gas typically monitored in mines?

The correct answer is carbon dioxide, as it is not one of the primary gases typically monitored in coal mines, where the focus is often on more hazardous gases. In mining environments, the primary gases that pose immediate risks to miners' safety are methane and oxygen. Methane is a highly flammable gas that can accumulate in mine shafts, creating explosive atmospheres. Oxygen levels are monitored to ensure that there is enough breathable air for miners.

Hydrogen, while not as commonly associated with coal mining as methane, can still be a concern in certain mining operations, particularly those involving specific chemical reactions or processes. Although it may not be a predominant gas in all mines, it is sometimes monitored in particular contexts.

Conversely, carbon dioxide is typically not a gas that is monitored specifically for its immediate dangers in mining, especially when compared to the direct risks posed by methane and the necessity of maintaining sufficient oxygen levels. Therefore, monitoring carbon dioxide is less critical in the typical context of mine safety compared to the other gases listed.