Why are a very large number of alveoli present in the lungs?

Amount of $CO_{2}$ in expired air is about (in $\%$)

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:

Which of the following statements is correct regarding the diffusion capacity of gases?

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

Which of the following is true for the exchange part of the respiratory system?