Which one of these gases is considered as an air pollutant, even if it is present in the air in small amounts?

Options

A. Oxygen
B. Nitrogen
C. Carbon dioxide
D. Carbon monoxide

Explanation

**Step-by-Step Solution:**

1. **Identify the Gases Listed**: The question provides four gases: Oxygen, Nitrogen, Carbon Dioxide, and Carbon Monoxide.

2. **Evaluate Each Gas**:
- **Oxygen**: Essential for life and required for respiration. It is not considered an air pollutant.
- **Nitrogen**: Makes up about 78% of the atmosphere and plays a crucial role in the nitrogen cycle. It is not an unwanted substance and thus not an air pollutant.
- **Carbon Dioxide**: While it is a greenhouse gas and can contribute to global warming when present in high concentrations, it is not harmful in small amounts. Therefore, it is not classified as an air pollutant.
- **Carbon Monoxide**: This gas is produced from the incomplete combustion of fossil fuels. It is harmful even in small amounts as it reduces the oxygen-carrying capacity of blood by forming carboxyhemoglobin, which is ineffective for oxygen transport.

3. **Conclusion**: Based on the evaluation, Carbon Monoxide is the gas that is considered an air pollutant, even when present in small amounts.

4. **Final Answer**: Carbon Monoxide.

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