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: Social & Preventive Medicine
[{'content': 'In a study in the UK, an association was found between the sale of antiarrhythmic drug and an increase in deaths due to asthma. This is an example of- \nSelect and explain. \nA. Ecological study, B. Cohort study, C. Case reference study, D. Experimental study', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]