Photochemical smog consists of excessive amount of X, in addition to aldehydes, ketones, peroxyacetyl nitrate (PAN), and so forth X is:

Options

A. CO
B. `CH_(4)`
C. `O_(3)`
D. `CO_(2)`

Explanation

To solve the question regarding the composition of photochemical smog, we need to identify the substance represented by "X" in the context of the pollutants mentioned.

### Step-by-Step Solution:

1. **Understanding Photochemical Smog**:
- Photochemical smog is a type of air pollution that occurs when sunlight reacts with nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the atmosphere. This reaction is facilitated by ultraviolet (UV) radiation.

2. **Identifying the Components**:
- The question states that photochemical smog consists of excessive amounts of "X," along with aldehydes, ketones, and peroxyacetyl nitrate (PAN).
- The major components of photochemical smog include nitrogen dioxide (NO2), ozone (O3), and various organic compounds.

3. **Role of Ozone**:
- During the photochemical reactions, ozone is formed as a secondary pollutant. It is created when nascent oxygen (produced from the photolysis of nitrogen dioxide) reacts with molecular oxygen (O2).
- Ozone is a significant component of photochemical smog and contributes to its harmful effects.

4. **Conclusion**:
- Given that "X" is a major component of photochemical smog along with aldehydes, ketones, and PAN, we can conclude that "X" is ozone (O3).

### Final Answer:
X is ozone (O3).

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