Gaseous movement into and out of the plants occurs indirectly through

Which of the following is correct regarding the partial pressures of gases ($PO_2$ and $PCO_2$) in different parts of the human respiratory system?

Solubility of $CO_2$ is . . . . . . times higher than that of $O_2$.

Partial pressure for oxygen and $CO_2$ is represented as

What is the percentage of oxygen in expired air (in $\%$)?

Diffusion of gases occurs in the alveolar region only and not in the other parts of the respiratory system. Why?