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': 'Ohotolidine test is used to determine- \nSelect and explain. \nA. Nitrates in water, B. Nitrites chlorine in water, C. Free and combined chlorine in water, D. Ammonia content in water', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Oho-toluidine test is used to determine both free and residual chlorine with speed and accuracy. It is done using oho-toluidine dissolved in 10 per cent solution of hydrochloric acid. When added to water containing chlorine it turns yellow and its intensity varies with the concentration of the gas. Oho-toluidine Arsenite test is used to determine free and residual chlorine separately. (PARK TB (25th edition pg 775)', 'role': 'assistant'}]