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

Q: The newest, outer layers of secondary xylem are called sapwood.

A: What are the newest, outer layers of secondary xylem called?
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Q: The cardiovascular system in the human body consists of the heart, a network of blood vessels, and blood.

A: Which system in the human body consists of the heart, a network of blood vessels, and blood?
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Q: Poles parts of the planet does solar energy have difficulty reaching.

A:
What parts of the planet does solar energy have difficulty reaching?
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