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 is false about cohort study - 
Select and explain. 
A. Incidence can be measured, B. Used to study chronic diseases, C. Expensive, D. Always prospective
D: Always prospective
Answer: D: Always prospective 
Explanation: None