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

Example Input: Economic equilibrium occurs where the _ lines intersect.
Example Output: two

Example Input: Older caterpillars have _ different color forms.
Example Output: two

Example Input: Antibodies exist as one or more copies of a Y-shaped unit, composed of _ polypeptide chains.
Example Output:
four