Discussion
When biological oxygen demand becomes very high ?
Answer: C
When BOD becomes very high, the water becomes anaerobic.
Biological oxygen demand is the amount of dissolved oxygen that aerobic microbes consume while breaking down the organic matter in a water sample. A very high BOD means a heavy organic load, and microbial decomposition uses up the dissolved oxygen faster than it is replaced. The water becomes oxygen-depleted, fish die, and anaerobic decomposition takes over.
So dissolved oxygen falls rather than rises, and conditions turn against aerobic microbes. Microbial numbers are high, not low, when BOD is high, which rules out the last option.
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