Q: Birds have light-weight bones that are filled with what?


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

Birds have light-weight bones that are filled with air. They also lack a jaw, which in many vertebrates is a dense, heavy bone with many teeth. Instead, birds have a light-weight keratin beak without teeth.

 Choices:

- water

- proteins

- blood

- air


A:
air