The figure given below shows a small part of human lung where exchange of gases takes place. In which one of the options given below, the one part A, B, C or D is correctly identified along with its function, <br>

Question

( A ) A : alveolar cavity – main site of exchange of respiratory gases
( B ) B : red blood cell – transport of $CO_2$ mainly
( C ) D : Capillary wall – exchange of $O_2$ and $CO_2$ takes place here
( D ) C : arterial capillary – passes oxygen to tissues