Which of the following statements is true about gas diffusion across respiratory membranes?

The possible answers are :
a.) Gases diffuse from areas of low concentration to areas of high concentration. 
b.) Gases diffuse from areas of high concentration to areas of low concentration. 
c.) Gases diffuse between areas of equal concentration. 
d.) Concentration has no effect on gas diffusion

The correct answer is b.) Gases diffuse from areas of high concentration to areas of low concentration. 
Gas concentrations tend to form an equilibrium over time. This means that the gas molecules diffuse from the area of high concentration to the area of low concentration in order to make the two concentration equal.