Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

[Q]: Plasma membranes encloses and defines the borders of cells.
[A]: What encloses and defines the borders of cells?


[Q]: A(n) an ionic bond forms when the metal sodium gives up an electron to the nonmetal chlorine.
[A]: What forms when the metal sodium gives up an electron to the nonmetal chlorine?


[Q]: The brain stem monitor(s) the level of carbon dioxide in the blood.
[A]:
What monitors the level of carbon dioxide in the blood?