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
[{'content': 'Prevalance of cataract at one point of time can be determined by - \nSelect and explain. \nA. Longitudinal study, B. Cross-sectional study, C. Surveillance, D. Coho study', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Cross-sectional study o This queston is straight forward, Prevelance is obtained by cross-sectional study and therefore cross-sectional study is also known as prevalence study.", 'role': 'assistant'}]