Which of the following is a secondary air pollutant?

Options

A. Hydrocarbons
B. smog
C. particulate matter
D. automobile exhausts

Answer

To determine which of the following is a secondary air pollutant, we need to understand the difference between primary and secondary air pollutants.

### Step-by-Step Solution:

1. **Define Primary Air Pollutants**: Primary air pollutants are substances that are directly emitted into the atmosphere from various sources. These include emissions from vehicles, industrial processes, and the burning of fossil fuels.

2. **Define Secondary Air Pollutants**: Secondary air pollutants are not directly emitted. Instead, they form in the atmosphere through chemical reactions involving primary pollutants. This means that secondary pollutants are the result of interactions between primary pollutants and other atmospheric components.

3. **Identify the Options**: The options given are:
- Hydrocarbons
- Smog
- Particulate Matter
- Automobile Exhaust

4. **Analyze Each Option**:
- **Hydrocarbons**: These are typically considered primary pollutants as they are emitted directly from sources like vehicle exhaust.
- **Smog**: This is a type of air pollution that results from the reaction of primary pollutants (like hydrocarbons and nitrogen oxides) in the presence of sunlight. Therefore, smog is a secondary air pollutant.
- **Particulate Matter**: This can be both primary and secondary, but it is often emitted directly from sources like construction sites, unpaved roads, and combustion processes.
- **Automobile Exhaust**: This is a primary pollutant as it is directly emitted from vehicles.

5. **Conclusion**: Based on the definitions and analysis, the correct answer is **B. Smog**, as it is a secondary air pollutant formed from the reaction of primary pollutants.

### Final Answer:
**B. Smog**