In the alveoli of the lungs, air crosses into and out of the capillaries by the process of diffusion because the oxygen moves from higher concentration region to lower concentration region.
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. This process requires no energy.
So we can conclude that diffusion is the right answer.
Learn more about diffusion here: https://brainly.com/question/94094
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