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': 'Which is not an epidemiological method of population survey? \nSelect and explain. \nA. Twin studies, B. Ecological studies, C. Correlational studies, D. Community trials', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]