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]: Pangolins walk on all _ of their stout limbs, which are adapted for digging.
[A]: four


[Q]: Malaria kills up to _ million people each year.
[A]:
two