Given a scientific question, generate a correct answer to it.

[EX Q]: In contrast to mammals and birds, freshwater fishes produce large volumes of very dilute what?
[EX A]: urine

[EX Q]: What is the main ingredient of mothballs?
[EX A]: hydrocarbon naphthalene

[EX Q]: The partial pressure of each gas in a mixture is proportional to its what?
[EX A]:
mole fraction