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': 'Prevalence of Vitamin A deficiency in a community is assessed as \nSelect and explain. \nA. Night blindness-10%, B. Corneal ulcer-0.01%, C. Bitot spots-0.5%, D. Decreased serum retinol level-0.05%', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]