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

Q: Oxygen gas is given off by plants.

A: Which gas is given off by plants?
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Q: Force times distance is the equation for work.

A: Force times distance is the equation for what?
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Q: Currently, the only known stable systems undergoing fusion are interiors of stars.

A:
Currently, the only known stable systems undergoing fusion are what?
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