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

Q: The water in some springs are hot because they're heated by hot magma.

A: The water in some springs are hot because they're heated by what?
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Q: Hemodialysis has a patient’s blood pass through a length of tubing that travels through an artificial kidney machine.

A: Which form of dialysis has a patient’s blood pass through a length of tubing that travels through an artificial kidney machine?
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Q: Deep currents are caused by differences in density at the top and bottom of the ocean.

A:
What is caused by differences in density at the top and bottom of the ocean?
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