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

Mammalian lungs have an enormous number of minute alveoli (air sacs). This is to allow:

The diffusion membrane of the lungs,through which the exchange of gases occurs,is made up of which of the following layers?

Exchange of gases in lungs occurs through

Partial pressure for oxygen and $CO_2$ is represented as

$A$: Diffusion of carbon dioxide is $20$ times faster than oxygen.
$R$: It is due to difference in partial pressure as well as solubility of diffusing gases.