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

The solubility of $CO_{2}$ in the blood is

Given below are two statements.
Statement $I$: Respiratory membrane is composed of two layers of squamous epithelia,each resting on a thick basement membrane.
Statement $II$: The diffusion of a respiratory gas takes place from lower partial pressure to higher partial pressure of the same.
In the light of above statements,choose the most appropriate answer from the options given below:

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

Body cells must continuously receive $P$ and release $Q$ produced by the cells. What are $P$ and $Q$ in the given statement?
$P \quad Q$

Explain: $(1)$ Exchange of gases by alveolus surface. $(2)$ Exchange of gases in tissues.