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
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: 
Select and explain. 
A. Cohort study., B. Case reference study., C. Ecological study., D. Experimental study.
C: Ecological study.
Answer: C: Ecological study. 
Explanation: None