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

Q: Potential energy type of energy is possessed by autumn leaves before they fall.

A: What type of energy is possessed by autumn leaves before they fall?
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Q: The nuclear envelope is a double-layered plasma membrane like the cell membrane, although without membrane proteins.

A: The nuclear envelope is a double-layered plasma membrane like the cell membrane, although without what?
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Q: Because the current is alternating, the magnetic field of the iron core keeps doing reversing.

A:
Because the current is alternating, the magnetic field of the iron core keeps doing what?
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