Answer:
C or the third answer choice
Step-by-step explanation:
If algae excessively grows on the tops of the water, it can block sunlight. That will make the underwater plants die, and the animals that eat those underwater plants will slowly have less to eat if they can't adapt quickly enough. When the algae dies, it takes oxygen with it. If it take all of the oxygen in the water, that whole ecosystem could potentially die as well.