The two sources of pollution that can decrease the oxygen level of water are:
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Animal manure - When animal manure enters water bodies, it can lead to nutrient pollution, causing algal blooms. When these algae die and decompose, the process consumes oxygen, leading to hypoxic conditions (low oxygen levels).
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Plant fertilizer - Similar to animal manure, fertilizers contain nutrients (particularly nitrogen and phosphorus) that can also promote algal blooms in water bodies. The decomposition of excess algal growth depletes oxygen levels in the water.
Toxic metals, gasoline, and radioactive substances can be harmful, but they do not directly cause a decrease in oxygen levels in water like the other two options.