Which statement best describes the human respiratory system? A) It is composed of a network of moist passageways that permit air to flow from the external environment to the lungs. B) Each cell of the human body is in direct contact with the external environment, and gas exchange occurs by diffusion. C) The external body surface is kept moist to allow for gas exchange. D) Gases diffuse across membranes on both the external and internal surfaces of the body.