The two sources of pollution that decrease the oxygen level of water are:
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Plant fertilizer - When fertilizers enter water bodies, they can cause algal blooms, which deplete oxygen levels when the algae die and decay.
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Animal manure - Similar to fertilizers, animal manure can also lead to nutrient pollution, resulting in algal blooms and subsequent oxygen depletion when the blooms decay.
Thus, the correct answers are plant fertilizer and animal manure.