A Dental Student Wants to Test If Fluoride Is an Effective

A dental student wants to test if fluoride is an effective additive against tooth decay. The student studies tooth decay in a population of people who live in neighborhoods supplied with fluoridated water. What would be an effective control group to ask this question?

A) Individuals with access to differing amounts of fluoride in the water.
B) Individuals with access to fluoridated water.
C) Individuals who have fluoride added to their toothpaste but not their water.
D) Individuals with access to water with no fluoride added.