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
Most difficult criterion on establishing a causal association in the aetiology of a disease is - 
Select and explain. 
A. Temporality, B. Strength of association, C. Specificity of association, D. Biological plausibility
C: Specificity of association
Answer: C: Specificity of association 
Explanation: Park's textbook of preventive and social medicine 23rd edition. *Specificity is the most difficult criterion to establish not only in chronic disease but also in acute diseases and conditions.