The following zones are formed in a polluted river:

1. zone of clear water

2. zone of active decomposition

3. zone of recovery

4. zone of pollution

The correct sequence in which these zones occur progressively downstream in a polluted river is

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

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