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: Lichens are not a single organism, but rather an example of a mutualism.

Answer: Lichens are not a single organism, but rather an example of a what?

Question: Gastro-esophageal sphincter sphincter is located at the stomach end of the esophagus and opens to allow the bolus to enter the stomach.

Answer: Which sphincter is located at the stomach end of the esophagus and opens to allow the bolus to enter the stomach?

Question: Population is the unit of evolution.

Answer:
What is the unit of evolution?