The two correct answers that decrease the oxygen level of water are:
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Animal manure - High levels of animal manure can introduce nutrients into water bodies, leading to algal blooms that deplete oxygen as they decay.
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Plant fertilizer - Like animal manure, fertilizers can cause nutrient runoff into water bodies, leading to similar oxygen depletion due to algal blooms.
Gasoline, radioactive substances, and toxic metals do not directly decrease the oxygen levels in water.