Given a scientific question, generate a correct answer to it.

[Q]: What is contained in the cavity of a long bone's central shaft?
[A]: marrow

[Q]: Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur are basic elements that form what type of compounds, which are vital to life?
[A]: organic compounds

[Q]: Many protists form what kind of beneficial association with other species?
[A]:
symbiotic