Q: The lungs of mammals are unique in having what?


Read this paragraph and choose the correct option from the provided answers:

The lungs of mammals are unique in having alveoli . These are tiny, sac-like structures. Each alveolus is surrounded by a network of very small blood vessels (see Figure below ). Because there are millions of alveoli in each lung, they greatly increase the surface area for gas exchange between the lungs and bloodstream. Human lungs, for example, contain about 300 million alveoli. They give the lungs a total surface area for gas exchange of up to 90 square meters (968 square feet). That’s about as much surface area as one side of a volleyball court!.

 Choices:

- alveolar

- glands

- acini

- alveoli


A:
alveoli