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': 'Effect of Pyrethrum on mosquitoes is described as – \nSelect and explain. \nA. Residual, B. Repellent, C. Knock down, D. Contact', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]