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': 'In a UK study, it was found that there were more deaths from asthma than the sale of anti-asthma drugs. This is an example of: \nSelect and explain. \nA. Cohort study., B. Case reference study., C. Ecological study., D. Experimental study.', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]