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': "What is the reagent used in Horrock's apparatus for determining chlorine demand of water – \nSelect and explain. \nA. Orthotoluidine, B. Methylene blue, C. Starch iodide indicator, D. Sodium bicarbonate", 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]