In which zone of pollution in a river stream the Dissolved Oxygen level is minimum?
- AZone of degradation
- BZone of active decompositionCorrect
- CZone of recovery
- DZone of cleaner water
Solution
Concept
Zones of river pollution
Step-by-step solution
1. Zone of degradation: DO starts falling. 2. Zone of active decomposition: DO is lowest (may reach zero). 3. Zone of recovery: DO builds up again. 4. Zone of cleaner water: DO near saturation.
Explanation
The sag point lies in the active decomposition zone.
Why the other options are wrong
DO is higher in the other zones.
Shortcut
Minimum DO → active decomposition.
Correct answer: B) Zone of active decomposition