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': 'Percentage of para-para isomer in DDT is: \nSelect and explain. \nA. 20-30%, B. 40-50%, C. 60-70%, D. 70-80%', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 40-50%', 'role': 'assistant'}]