In this task, find the most appropriate number to replace the blank (indicated with _ ) and express it in words.

[EX Q]: Mental illness affects _ out of four families in America.
[EX A]: one

[EX Q]: Temperatures occasionally fall below _ but only last a very short time.
[EX A]: zero

[EX Q]: Common irons is a mixture of _ isotopes.
[EX A]:
four