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

Input: Consider Input: Optical isomers differ in the arrangement of _ groups around a chiral carbon.

Output: four

Input: Consider Input: Postpartum blues are common during the _ months after childbirth.

Output: three

Input: Consider Input: Women taking vitex for _ months appear to have more success at becoming pregnant.
Output: three