collection of waste material breakdown of materials diffusion of material storage of materials
I gave my students this multiple choice question: Capillary walls are only one layer of cells thick. Passage of oxygen into and out of the capillaries is controlled most by which process?

Only one answer is correct among these 4 choices:
- collection of waste material
- breakdown of materials
- diffusion of material
- storage of materials

Could you tell me which one is correct?

diffusion of material