John visits the dentist every six months for a check-up. During his latest appointment, the dentist mentioned that several patients have been switching from manual toothbrushes to electric ones. John noticed that the waiting room magazine rack was full of advertisements for electric toothbrushes. Based on this, John claims that switching to an electric toothbrush is the main reason why people who use them get fewer cavities than those who use manual toothbrushes.