Most water companies in the United States add fluoride to tap water to help prevent cavities. Some dentists argue, however, that this practice actually causes more harm than good because people overestimate the protection afforded by the fluoride and do not take the proper steps to care for their teeth, such as brushing and flossing after every meal. If water companies did not add fluoride, the dentists claim, people would be forced to be more active in their dental hygiene and tooth decay would decline as a result. Which of the following, if true, most weakens the dentists' claims? ['Most Americans already brush their teeth daily.', 'Annual visits to the dentist are the most effective means of controlling tooth decay.', 'Most Americans are not aware that fluoride is added to tap water.', 'Fluoride is widely used in commercially available dental care products.']