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

[EX Q]: A pregnant woman should avoid toxic substances while pregnant.
[EX A]: What should a pregnant woman avoid while pregnant?

[EX Q]: Clear mucus is secreted before ejaculation to neutralize acidic urine remaining in the urethra.
[EX A]: What is secreted before ejaculation to neutralize acidic urine remaining in the urethra?

[EX Q]: Superconductors are materials with a resistivity of zero.
[EX A]:
Superconductors are materials with a resistivity of?