How does oxygen move from the respiratory system to the circulatory system?
O A. By moving across a synapse
O B. By pumping through the heart
O C. By circulating in an artery
D. By diffusion across a capillary wall

D. By diffusion across a capillary wall

Explanation:

D is the best answer because gas exchange takes place in our lungs. Our lungs have alveoli which have a network of tiny blood vessels called capillaries, here the O2 diffuses the capillary wall to enter the bloodstream, and similarly CO2 diffuses out and we exhale it our from our lungs.