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

Name the primary site of exchange of gases in our body?

The partial pressure of $O_{2}$ and $CO_{2}$ in atmospheric air as compared to those in alveolar air is:

Gas	Alveolar Air $(X/A)$	Atmospheric Air $(Y/B)$
$pO_{2}$	$X$	$Y$
$pCO_{2}$	$A$	$B$

Which of the following factors is most necessary for an efficient respiratory surface?

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 structure of the lungs is directly involved in $O_2 / CO_2$ exchange between air and blood capillary?