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

The low partial pressure of oxygen $(PO_2)$ in the body cells results in:

Identify $A, B$ and $C$ in the given diagram and choose the correct option accordingly.

The blood leaving the lungs has all its haemoglobin oxygenated. Why does it give up oxygen to the tissues?

What is the thickness of the diffusion membrane involved in the exchange of gases in the lungs?

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: