A polluted stream undergoes self-purification in four distinct zones:

1. Zone of clear water

2. Zone of active decompositon

3. Zone of degradation

4. Zone of recovery

The correct sequence of these zones is

- 1
  
  3, 4, 2, 1
- 2
  
  2, 3, 4, 1
- 3
  
  2, 4, 3, 1
- 4
  
  3, 2, 4, 1

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