Which air pollutant is not released by motor vehicles?

Options

A. `SO_2`
B. Fly ash
C. Hydrocarbons
D. CO

Explanation

To determine which air pollutant is not released by motor vehicles, we need to analyze the options provided: SO2, fly ash, hydrocarbons, and CO.

### Step-by-Step Solution:

1. **Identify the Air Pollutants**:
- We have four options:
- SO2 (Sulfur Dioxide)
- Fly Ash
- Hydrocarbons
- CO (Carbon Monoxide)

2. **Understand Motor Vehicle Emissions**:
- Motor vehicles typically emit a variety of pollutants, including:
- CO (Carbon Monoxide)
- Hydrocarbons
- CO2 (Carbon Dioxide)
- NOx (Nitrogen Oxides)
- However, SO2 is primarily produced from the burning of fossil fuels in power plants and industrial processes, not directly from motor vehicles.

3. **Analyze Each Option**:
- **SO2**: While it is a common air pollutant, it is not primarily emitted by motor vehicles.
- **Fly Ash**: This is a byproduct of burning coal in power plants and is not emitted by motor vehicles.
- **Hydrocarbons**: These are released from motor vehicles as they burn fuel.
- **CO**: This is also a common emission from motor vehicles.

4. **Conclusion**:
- Among the options, **fly ash** is the air pollutant that is not released by motor vehicles.

### Final Answer:
The air pollutant that is not released by motor vehicles is **fly ash**.

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