Answer:
b.) diffusion
Explanation:
Gas exchange is occur through the diffusion across the surface.
Diffusion:
It is a process in which gas exchange is occur through the concentration gradient. Gas molecules are moves from region of high concentration to the region of lower concentration.
Example:
In blood the concentration of oxygen is low and carbon dioxide is high while the air in lungs is high in oxygen concentration and low in carbon dioxide concentration. This concentration gradient leads to the exchange of gases in respiration. During respiration oxygen gas is intake while carbon dioxide is released.
