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

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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Q: The vascular structure functions as a framework that reinforces the shape of the leaf.

A: What functions as a framework that reinforces the shape of the leaf?
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Q: Viral infection of multicellular eukaryotes is usually limited to particular tissues.

A:
Which infection of multicellular eukaryotes is usually limited to particular tissues?
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