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

[EX Q]: Reverse osmosis uses three to ten gallons of water to produce _ gallon of drinking water.
[EX A]: one

[EX Q]: Masturbation tends to peak when children are between the ages of _ and five.
[EX A]: three

[EX Q]: Doctors spend _ years in medical school, so they're experts on human anatomy.
[EX A]:
four