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

Q: Light retains its original color under water because frequency remains the same when light is refracted.

A: Light retains its original color under water because what remains the same when light is refracted?
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Q: All enzymes have an optimal temperature and ph.

A: All enzymes have an optimal temperature and what?
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Q: Black light color light is actually the absence of electromagnetic waves in the visible spectrum, for humans.

A:
What color light is actually the absence of electromagnetic waves in the visible spectrum, for humans?
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