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
All are true about cohort studies except - 
Select and explain. 
A. Prospective, B. Useful for rare diseases, C. Necessary for incidence, D. Costly
B: Useful for rare diseases
Answer: B: Useful for rare diseases 
Explanation: None