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

Example Input: Polymorphism occurs in all species and the mandibles bear _ teeth.
Example Output: four

Example Input: Conserved residues make similar contacts in _ repressor-operator complexes.
Example Output: two

Example Input: Most infants become ready to try solid foods when they are between _ and six months of age.
Example Output:
four