Cnidaria and flatworms exchange gases and remove wastes by way of their:
A) open circulatory systems. B) closed circulatory systems. C) gastrovascular cavities. D) moist skin surfaces. E) respiratory and excretory systems.

The answer is D, moist skin surfaces.

Explanation:
Cnidaria and flatworms respirate and extreme through their skin via diffusion.

The return of the wolf to yellowstone national park will probably affect the bird population to some degree, not significantly. while wolves will hunt birds when necessary, they prefer to hunt larger prey. the bird population will most likely only be affected when hunting is scarce for the wolves. birds of prey will also probably not be affected as much as other birds who spend more time on the ground hunting for food as the tend to spend more time in the air or in trees since they do not need to stay on the ground for long due to the nature of the way they hunt.

The answer would be d. pili