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: Population is an important level of organization in ecology and the unit of microevolution.

A: What is an important level of organization in ecology and the unit of microevolution?
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Q: Mollusks can be divided into seven classes.

A:
Mollusks can be divided into how many classes?
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