Which of the following best describes what alveoli are?
a. they are the smallest tubes in the respiratory system.
b. they have a small amount of surface area.
c. they are found on the end of bronchioles.
d. they are comprised of millions of cells.

The correct answer is option c. They are found on the end of bronchioles.

The alveoli are the small sac like structure and are the actual site of the gaseous exchange between the blood and lungs.

The trachea is the main tube, which carries the air from the nostrils to the lungs. The trachea is further divided in to two bronchi, each bronchus penetrates each lung. The bronchi is divided into many bronchioles and at the end of the bronchioles, the sac like alveoli are present.  

c. They are found on the end of bronchioles.

Explanation:

just took test

d

explanation:

the tricuspid valve is present between right atrium and right ventricle