Given a scientific passage and an answer, generate a question for the given answer.

[Q]: Passage: Density is defined as the mass of an object divided by its volume. Propose a unit of density in terms of the fundamental SI units. Answer: volume
[A]: Density is defined as the mass of an object divided by its what?

[Q]: Passage: The Gaia hypothesis states that the biosphere is its own living organism. Answer: the gaia hypothesis
[A]: What hypothesis states that the biosphere is its own living organism

[Q]: Passage: Compact bone makes up the dense outer layer of bone. Its functional unit is the osteon . Compact bone is very hard and strong. Answer: osteon
[A]:
What is the functional unit of compact bone?