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

Example Input: Pets also track in pollen, and _ out of every ten people is allergic to cat or dog dander.
Example Output: one

Example Input: Morocco has _ perennial rivers and many dams have been built with potential for power generation.
Example Output: four

Example Input: Butterflies also carry _ wings, which are covered by colored scales.
Example Output:
four