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
As compared to a routine case-control study, nested case-control study avoids problems (in study design) related to - 
Select and explain. 
A. Temporal association, B. Confounding bias, C. Need for long follow up, D. Randomization
A: Temporal association
Answer: A: Temporal association 
Explanation: None