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

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


[Q]: Screws provide at least _ times the holding power of nails.
[A]: seven


[Q]: Friendships often go through _ phases, according to psychologists.
[A]:
four