In this task, you are given a question and an answer, you would be asked to create the sentence based on the Question-Answer provided. It should be contained within the Question-Answer provided.

[Q]: Question: What is the key to the demographic transition? Answer: reduced family size
[A]: Reduced family size is the key to the demographic transition.


[Q]: Question: What describes the amount of solute in a solution? Answer: tonicity
[A]: Tonicity describes the amount of solute in a solution.


[Q]: Question: Contributing to the blood-brain barrier is one of the jobs of glial cells, which support neurons in what system? Answer: central nervous system
[A]:
Contributing to the blood-brain barrier is one of the jobs of glial cells, central nervous system support neurons in what system.