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: Dental
[{'content': 'Residual chlorine of water should be: \nSelect and explain. \nA. 1 mg/ltr after 1 hr, B. 1 mg/ltr after ½ hr, C. 0.5 mg/ltr after 1 hr, D. 0.5 mg/ltr after ½ hr', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Principles of chlorination:\xa0\n\nThe "chlorine demand" of the water should be estimated. The chlorine demand of the water is the difference between the amount of chlorine added to the water and the amount of residual chlorine remaining at the end of a specific period of contact at a given temperature and pH of the water.\nThe minimum recommended concentration of free chlorine is 0.5 mg/liter for one hour. The free residual chlorine provides a margin of safety against subsequent microbial contamination which may occur during storage and distribution.\n\nEssentials of preventive and community dentistry\xa0\nSoben Peter\xa0\n5th edition', 'role': 'assistant'}]