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

[Q]: House wrens are small birds, four and a half to _ and a four inches in length.
[A]: five

[Q]: Retinoic acid regulates oral epithelial differentiation by _ mechanisms.
[A]: two

[Q]: Salicylic acid has a half life of about _ hours at therapeutic doses.
[A]:
four