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

[EX Q]: Rams can be policerate, often producing _ horns.
[EX A]: four

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

[EX Q]: R contains _ eigenvectors.
[EX A]:
four