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': 'The following statement about DDT is false: \nSelect and explain. \nA. Pyrethrum has synergistic action, B. It is a contact poison, C. Immediately kills the prey, D. Residual effect lasts for 18 months', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Immediately kills the prey Repeat from May 11 DDT does not immediately kill the insects. It takes several hours to kill them.', 'role': 'assistant'}]