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': "Horrok's apparatus determines Chloride which has to have a holding level of – \nSelect and explain. \nA. 0.1 mg/L, B. 1 mg/L, C. 0.01mg /L, D. 0.5 mg/L", 'role': 'user'}, {'content': "Answer: D \nExplanation: Horrock's apparatus is used to find out the dose of bleaching powder required for disinfection of water by chlorination.\nThe principle in chlorination is to ensure a free residual chlorine of 0.5 mg/litre at the end of one hour contact.", 'role': 'assistant'}]