The correct answer is B. They have well-developed respiratory structures.

Annelids, such as earthworms, have a more advanced respiratory system compared to flatworms. They possess Setae (bristles) that increase the surface area for gas exchange, a well-developed circulatory system with blood capillaries that facilitate oxygen transport as well as a moist, thin cuticle that allows for gas diffusion.

These specialized respiratory structures enable annelids to exchange gases more efficiently than flatworms, which rely on simple diffusion across their body surface.