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
To test the association between risk factor and disease, which of the following is the weakest study design - 
Select and explain. 
A. Case-control study, B. Ecological study, C. Cohort study, D. Cross-sectional study
B: Ecological study
Answer: B: Ecological study 
Explanation: None