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JEE Main levelSingle correct optionDifficulty 2 of 3ev132

Just downstream of the point where untreated sewage enters a river, the water shows:

  1. Aa fall in BOD and a rise in dissolved oxygen
  2. Ba rise in BOD and a fall in dissolved oxygen
  3. Cno change in either BOD or dissolved oxygen
  4. Da rise in both BOD and dissolved oxygen

Answer: B

Sewage adds biodegradable organic matter, so BOD increases. Aerobic microorganisms consume dissolved oxygen while oxidising it, so dissolved oxygen falls.
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