All animals have a system for exchanging carbon dioxide and oxygen with their surroundings. What do all of these systems have in common?

A. All involve the active transport of gases across membranes.
B. All include airways, such as the trachea, that carry air to the mouth.
C. All involve gas diffusion across moist, selectively permeable membranes.
D. All involve organs or tissues that are dedicated to gas exchange and fulfill no other functions.