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

Input: Consider Input: Older caterpillars have _ different color forms.

Output: two

Input: Consider Input: Babies can range in age from newborn up to _ years old.

Output: three

Input: Consider Input: Blood flows in one direction through the four chambers with the aid of the _ heart valves.
Output: four