Secondary pollutants is/are:-
(a) Brown air (b) Grey air
(c) Acid Rain (d) DDT

Options

A. a,b and c are correct
B. a and b are correct
C. b and d are correct
D. a and c are correct

Answer

To determine which of the given options are secondary pollutants, we need to understand what secondary pollutants are and how they are formed.

### Step-by-Step Solution:

1. **Define Secondary Pollutants**:
- Secondary pollutants are not directly emitted into the atmosphere. Instead, they are formed through chemical reactions between primary pollutants and other atmospheric components.

2. **Analyze Each Option**:
- **(a) Brown Air**: This is also known as photochemical smog, which is formed when sunlight reacts with pollutants like nitrogen oxides (NOx) and volatile organic compounds (VOCs). Hence, brown air is a secondary pollutant.

- **(b) Grey Air**: This refers to a type of smog that is primarily composed of particulate matter and is associated with the burning of fossil fuels. It is considered an incomplete combustion product and can also be classified as a secondary pollutant due to its formation from primary pollutants.

- **(c) Acid Rain**: Acid rain is formed when sulfur dioxide (SO2) and nitrogen oxides (NOx) react with water vapor in the atmosphere to produce sulfuric and nitric acids. Therefore, acid rain is also a secondary pollutant.

- **(d) DDT**: DDT (Dichloro-Diphenyl-Trichloroethane) is a synthetic pesticide that is directly applied to the environment and is not formed through chemical reactions in the atmosphere. Thus, DDT is not a secondary pollutant.

3. **Conclusion**:
- Based on the analysis, the correct answers are (a) Brown air, (b) Grey air, and (c) Acid Rain. Therefore, the correct option is that secondary pollutants include A, B, and C.

### Final Answer:
Secondary pollutants are (a) Brown air, (b) Grey air, and (c) Acid Rain.

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