Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
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Question: The sun is considered the center of the solar system.

Answer: What is considered the center of the solar system?

Question: Sodium-potassium pumps help nerve cells establish a voltage across their plasma membranes.

Answer: Sodium-potassium pumps help nerve cells establish a voltage across their what?

Question: The octet rule is a result of trends in energies and is useful in explaining why atoms form the ions that they do.

Answer:
Which rule is a result of trends in energies and is useful in explaining why atoms form the ions that they do?