Researchers conducted questionnaire surveys and cognitive tests on 1003 elderly people over the age of 70 in a country. The results of the survey found that the elderly who used to drink more than 2 cups of green tea per day and the elderly who used to drink less than 3 cups of green tea per week In contrast, the former is about half as likely to suffer from cognitive impairment as the latter, and regular drinking of juice and coffee has no effect. Researchers believe that regular drinking of green tea may delay brain aging and reduce the risk of Alzheimer's. Which of the following, if true, would most challenge the above conclusion? ['Those in the survey who are accustomed to drinking more than 2 cups of green tea a day are healthier.', 'Seniors who drink less than 3 cups of green tea per week in the survey prefer to drink juice and coffee.', 'Studies have shown that green tea contains caffeine and other substances, and long-term drinking can lead to reduced sleep quality.', 'The elderly in the survey who drink less than 3 cups of green tea per week also do not like drinking water, and brain dehydration is the main cause of dementia.']