This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Biostatistics, Subject name: Social & Preventive Medicine
Association of two variables explained by third variable 
Select and explain. 
A. Spurious association, B. Indirect association, C. Direct association, D. Causal association
B: Indirect association
Answer: B: Indirect association 
Explanation: Ref:Park's Textbook of Preventive and Social Medicine 25th Ed