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
A study was done where the sale of anti-asthma drug was recorded and the number of deaths because of asthma was measured over a period of 15 years (1975-1990). This is which type of study. 
Select and explain. 
A. Ecological, B. Case reference, C. Experimental, D. Psephology
A: Ecological
Answer: A: Ecological 
Explanation: None