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: Most hormones are produced by the endocrine system.

Answer: Most hormones are produced by which human organ system?

Question: Oxygen gas is given off by plants.

Answer: Which gas is given off by plants?

Question: All types of cells are enclosed by a membrane.

Answer:
All types of cells are enclosed by what?