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

[Q]: Control rods play an important role in the modulation of the nuclear chain reaction.
[A]: What play an important role in the modulation of the nuclear chain reaction?


[Q]: The cells in the follicle undergo physical changes and produce a structure called a corpus luteum.
[A]: The cells in the follicle undergo physical changes and produce a structure called a what?


[Q]: Electrons in the outer energy level of an atom are called valence electrons.
[A]:
What are electrons in the outer energy level of an atom called?