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': 'Which one of the following is a secondary pollutant? \nSelect and explain. \nA. Benzene, B. Nitrogen oxide, C. Ozone, D. Sulphur dioxide', 'role': 'user'}, {'content': "Answer: C \nExplanation: Secondary air pollutants are formed within the atmosphere itself, they arise from chemical reactions of primary pollutants, most familiar example is ozone. Park's Textbook of Preventive and Social Medicine, 25th Edition, Pg 793", 'role': 'assistant'}]