This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
[{'content': 'Karlstad studies were held in: \nSelect and explain. \nA. Netherlands, B. London, C. Sweden, D. Finland', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Since artiﬁcial ﬂuoridation is not permitted in Sweden a number of other initiatives were introduced in 1971 as part of the "Karlstad studies" to evaluate the separate and combined eﬀects of diﬀerent\xa0plaque control programs.\xa0\nThe eﬃcacy of professional tooth cleaning was extensively investigated\xa0for the ﬁrst time in the so-called Karlstad studies.', 'role': 'assistant'}]