Which of the Following Would NOT Be Considered a Step

Which of the following would NOT be considered a step in gas transport and/or gas exchange?

A) the exhalation of CO₂ by a human into his or her environment
B) the transport of O₂ in the circulatory system of a frog
C) the diffusion of CO₂ out of mouse cells into a mouse's bloodstream
D) the diffusion of O₂ from the lungs of a horse into its bloodstream
E) All of these choices are correct.