Oxygen-demanding wastes
ADecrease the oxygen level in water ✓ Correct
BIncrease the oxygen level in water
CDo not change the oxygen level in water
DIncrease the temperature
Correct answer: (A) Decrease the oxygen level in water
Explanation
Oxygen-demanding wastes decrease dissolved oxygen as microbes consume it while decomposing organic matter.
This is measured as biochemical oxygen demand in polluted water.
They do not add oxygen, so the idea that they raise the oxygen level is wrong.
Because decomposition steadily uses oxygen, the level cannot stay unchanged.
Their effect is on oxygen, not primarily on water temperature.
Severe oxygen depletion can create dead zones where aquatic life cannot survive.
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