Have a pseudocoelom and are the simplest animals

The question is not complete. It should be " Have a pseudocoelom and are the simplest animals with digestive tracts.

The answer is Nematodes.

Explanation:

Nematodes are also called roundworm. They are group of diverse worms that have complete digestive tracts with two openings. They have nearly adapted to all ecosystems. The have pseudocoelom and are the simplest animals. They are bilaterally symmetrical and they are cylindrical in shape. They exhibit tissue level of organisation. E.g thread worm.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."

the answer is:

carbon dioxide