Answer: The answer is D: a digestive system with a single opening.
Cnidarians and flatworms have a digestive system with a single opening.
Explanation: Cnidarians are species of aquatic animals. They are mostly marine organisms that live in both marine and freshwater where they get food. Their respiration and digestion takes place through one orifice and a body cavity.
Flatworms are simple soft-bodied invertebrate animals. Their bodies are not divided into segments. Flatworms depend on a large surface in order to allow oxygen and nutrition to diffuse into their bodies. They have a digestive system with a single opening. Their habitation are in marine, freshwater, and damp terrestrial environments.
