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': 'Orthotoluidine Arsenite test determines – \nSelect and explain. \nA. Free residual chlorine, B. Combined chlorine, C. Free and combined chlorine separately, D. Free and combined chlorine together', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Orthotolidine -Aresenite (OTA) Test\n\nThis is a modification of the OT test to determine the free and combined chlorine residual separately.\nFurther, the errors caused by the presence of interfering substances such as nittrine, iron and manganese all of which produce a yellow colour with orthotolidine, are overcome by the OTA test.\n\nAlso know\n\nQuickest and simplest method for free residual chlorine → Diethyl parpaphenylene Diamine (DPD) indicator test.\nFree residual chlorine is a measure of the portability of water.', 'role': 'assistant'}]