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

[EX Q]: People have _ kidneys.
[EX A]: two

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

[EX Q]: Japanese archaeologists recognize, in general terms, _ prehistoric and four historic periods.
[EX A]:
four