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
Which of the following research methods include only people who are initially free of the disease of interest? 
Select and explain. 
A. Case series repo, B. Cross-sectional, C. Case-control study, D. Coho study
D: Coho study
Answer: D: Coho study 
Explanation: Only coho study moves forward with time. Here we study groups, who do not have the disease of interest initially but develops with time or not. Ref: Park's textbook of Preventive and Social Medicine, 21st edition, page 67-76