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: Prokaryotes group of simple organisms recycles nutrients and has been on earth since long before multicellular life appeared.

Answer: What group of simple organisms recycles nutrients and has been on earth since long before multicellular life appeared?


Question: Natural resource type of resource is limited in supply and cannot be replaced except over millions of years.

Answer: What type of resource is limited in supply and cannot be replaced except over millions of years?


Question: When electrons return to a lower energy level, they emit the excess energy in the form of light.

Answer:
When electrons return to a lower energy level, they emit the excess energy in the form of what?