The two sources of pollution that decrease the oxygen level of water are:
C. plant fertilizer - Fertilizers can lead to nutrient pollution, causing algal blooms. When these algae die and decompose, the process consumes oxygen in the water, leading to hypoxia (low oxygen levels).
D. animal manure - Like fertilizers, animal manure can also contribute to nutrient pollution, resulting in similar algal blooms that deplete oxygen in water bodies.
So, the correct answers are C and D.