instruction:
In this task, find the most appropriate number to replace the blank (indicated with _ ) and express it in words.
question:
Retinoic acid regulates oral epithelial differentiation by _ mechanisms.
answer:
two


question:
Plasma levels fall by about a half within three to _ hours.
answer:
four


question:
Some people have rubella, but show _ symptoms at all.
answer:

no