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: 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: Photoautotrophs and chemoautotrophs are two basic types of autotrophs.

Answer: Photoautotrophs and chemoautotrophs are two basic types of what?

Question: Complete and incomplete are the two types of digestive systems invertebrates may have.

Answer:
What are the two types of digestive systems invertebrates may have?