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 combustion of propane gas produces carbon dioxide and water vapor.

Answer: The combustion of propane gas produces carbon dioxide and what else?

Question: Mucous does the esophagus produce for lubrication.

Answer: What does the esophagus produce for lubrication?

Question: Studies have shown that not only does reproduction have a cost as far as sperm long male fruit flies live, but also fruit flies that have already mated several times have limited amounts of this.

Answer:
Studies have shown that not only does reproduction have a cost as far as how long male fruit flies live, but also fruit flies that have already mated several times have limited amounts of this?