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

Example Input: Humans host _ species of louse, the head louse , the body louse and the pubic louse .
Example Output: three

Example Input: Melanocytes are dendritic and are distributed roughly _ per six basal cells.
Example Output: one

Example Input: Babies first laugh when they are as young as _ to four months old.
Example Output:
three