Which of the following is not considered to be a pollutant ?

Options

A. `NO_2`
B. `CO_2`
C. `O_3`
D. `C_x H_y`

Answer

To determine which of the following substances is not considered a pollutant, we need to analyze each option provided. The options are NO2, CO2, O3, O2, CX, and HY.

### Step-by-Step Solution:

1. **Identify the Options**: The options given are:
- NO2 (Nitrogen Dioxide)
- CO2 (Carbon Dioxide)
- O3 (Ozone)
- O2 (Oxygen)
- CX (a generic representation of a compound)
- HY (a generic representation of a compound)

2. **Define Pollutants**: A pollutant is typically defined as a substance that, when present in the environment in high concentrations, can cause harm to living organisms or the ecosystem.

3. **Analyze Each Option**:
- **NO2**: This is a nitrogen oxide and is considered a pollutant due to its harmful effects on health and the environment.
- **CO2**: Carbon dioxide is a greenhouse gas. At normal atmospheric concentrations (approximately 0.033%), it is not considered a pollutant. However, elevated levels can contribute to climate change and are considered pollutants.
- **O3**: Ozone at ground level is a pollutant and can cause respiratory issues and other health problems.
- **O2**: Oxygen is essential for life and is not considered a pollutant under normal atmospheric conditions.
- **CX and HY**: Without specific information about these compounds, we cannot definitively classify them, but they could potentially be pollutants depending on their nature.

4. **Conclusion**: Among the options, **CO2** is generally not considered a pollutant at its typical concentration in the atmosphere (0.033%). Therefore, it is the correct answer to the question.

### Final Answer:
**CO2 (Carbon Dioxide) is not considered to be a pollutant at normal atmospheric levels.**