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.

They are found at the end of bronchioles.

Explanation:

Alveolar sac can be defined as the space which helps in maximizing the surface area  for the exchange of gas. It allows the lungs to exchange the carbon dioxide and oxygen in between the lungs and bloodstream.

Further explanation:

The exchange of gases in the human body   takes place through the respiratory system. It starts with the nasal cavity and ends on the alveolar sac where the the actual exchange of gases takes place.

The carbon dioxide from the bloodstream is diffused into the lungs and the oxygen from the lungs is diffused into the blood of the body.

The alveolar sac is the structure which increases the surface area maximum times in order to allow maximum exchange of gas.

After the exchange of gases the oxygenated blood is  distributed to the various organs of the body and the deoxygenated  blood from the organs is brought back to the lungs for purification.

Learn more:

1. What is the function of alveolar sac in respiratory system? link (answered by user)

2. How does the structure of the alveoli allow for its function of gas exchange?link (answered by histrionicus)

Keywords

Respiratory system, alveolar sac, oxygen, carbon dioxide, deoxygenated, oxygenated.