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
Healthy worker effect is bias of which type? 
Select and explain. 
A. Selection bias, B. Recall bias, C. Confounding bias, D. Berksonian bias
A: Selection bias
Answer: A: Selection bias 
Explanation: None