This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
In a study in UK, an association was found between sale of antiarrythmic drug and an increase in deaths due to asthma. This is an example of- 
Select and explain. 
A. Ecological study, B. Coho study, C. Case reference study, D. Experimental study
A: Ecological study
Answer: A: Ecological study 
Explanation: .