How Is It That Animals Such as Hydras and Flatworms

How is it that animals such as hydras and flatworms do not have a traditional circulatory system,yet they are capable of gas exchange?

A) Hydra and flatworms exchange gases across the cells that line the gastrovascular cavity.
B) Hydra and flatworms exchange gases across the cells of their coelom.
C) Hydra and flatworms exchange gases across the cells of their gill cavity.
D) Hydra and flatworms exchange gases across the cells of their skin performing cutaneous respiration.
E) Hydra and flatworms exchange gases across the cells of their lungs.