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

Q: Ocean ridges formed by marine invertebrates living in warm shallow waters within the photic zone of the ocean are called coral reefs.

A: Ocean ridges formed by marine invertebrates living in warm shallow waters within the photic zone of the ocean are called what?
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Q: Oil is the most commonly used source of energy in the world.

A: What is the most commonly used source of energy in the world?
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Q: Gases have the lowest density.

A:
Which state of matter has the lowest density?
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