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

Example Input: Natural iron contains _ isotopes and isomers.
Example Output: four

Example Input: Muskrats are semi-aquatic and prefer locations with four to _ feet of water.
Example Output: six

Example Input: Keen eyesight is complemented by _ sensitive odour-detecting antennae.
Example Output:
two