The correct option is B Alveoli
∙Nostrils are the two external openings of the nasal cavity that allows air to enter the respiratory tract.
∙Trachea, also called the windpipe, is a cartilaginous tube that starts from the larynx and further divides into two air passages known as the bronchi. The air travels through the bronchi and enters the lungs.
∙The bronchi branches into bronchioles which open into tiny air sacs known as alveoli.
∙The alveoli are richly supplied with blood capillaries. Alveoli and capillaries both have very thin walls, which allow the exchange of oxygen and carbon dioxide to take place.