Aquatic plants that float on water in a pond can reproduce rapidly, covering the surface of the pond. These aquatic plants can impact the oxygen levels in the pond when when they die. Sometimes the oxygen levels are so severely impacted that there is no longer enough oxygen in the water for fish to survive.

Why is there not enough oxygen for the fish to survive when a large population of the aquatic plants die?
a. The bacteria use oxygen in the water as they decompose the plants.
b. The dead plants are covering the surface of the pond and blocking the sunlight.
c. The dead plants create a blanket on the top of the pond causing the water to warm up.
d. The plants produce carbon dioxide instead of oxygen when they die.

I think it is A but all of them seem correct