This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, 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: D \nExplanation: park's textbook of preventive and social medicine 23rd edition. *The 'technical' DDT contains 70-80% of para-para isomer,which is the most active fraction of DDT.", 'role': 'assistant'}]