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. Cohort study, C. Ecological study, D. Cross sectional study
C: Ecological study
Answer: C: Ecological study 
Explanation: None