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': 'Grand rapids & Muskegon study is based on \nSelect and explain. \nA. Field trial, B. Observational studies, C. Randomized controlled trials, D. Community trials', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]