Why are lichens called indicators of air pollution?

Explanation

### Step-by-Step Solution:

1. **Definition of Lichens**:
- Lichens are a symbiotic association between two organisms: fungi and algae.
- The fungi provide nutrients and water to the algae, while the algae perform photosynthesis to produce food.

2. **Sensitivity to Air Pollution**:
- Lichens are sensitive to air pollutants, particularly sulfur dioxide (SO2).
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