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

[EX Q]: The main advantage of urea is its very low toxicity.
[EX A]: The main advantage of urea is its very low what?

[EX Q]: Resonance is the term for the use of two or more lewis structures to represent the covalent bonding in a molecule.
[EX A]: What is the term for the use of two or more lewis structures to represent the covalent bonding in a molecule?

[EX Q]: The newest, outer layers of secondary xylem are called sapwood.
[EX A]:
What are the newest, outer layers of secondary xylem called?