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: Water contains 3 atoms.

Answer: How many atoms does water contain?


Question: Since the electric field lines point radially away from the charge, they are perpendicular to equipotential lines.

Answer:
Since the electric field lines point radially away from the charge, they are perpendicular to what?