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': 'All of the following are true about break point chlorination, except – \nSelect and explain. \nA. Free chlorine is released in water after break point chlorination, B. Chlorine demand is the amount needed to kill bacteria, oxidize organic matter and neutralize ammonia, C. 1 ppm free chlorine should be present in water after break point has reached, D. Contact period of 1 hour is necessary', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Principles of chlorination\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\xa0\n\nWater should be clear and free from turbidity. Turbidity impedes efficient chlorination.\nThe chlorine demand of the water should be estimated. It is the amount of chlorine that is needed to destroy bacteria and to oxidize all the organic matter and ammoniacal substances present in water. Chlorine demand of water is the difference between the amount of chlorine added to the water and the amount of residual chlorine remaining at the end of specific period of contact (usually 60 minutes) at a given temperature and pH of the water.\nThe point at which the chlorine demand of the water is met is called break point chlorination. If further chlorine is added beyond break point, free chlorine begins to appear in water.\nThe presence of free residual chlorine for a contact period of at least one hour is essential to kill bacteria and virus.\nThe minimum recommended concentration of free chlorine is 0.5mg/i for one hour. This free residual chlorine provides margin of safety against subsequent microbial contamination.\nThe sum of the chlorine demand of the specific water plus the free residual chlorine of .5 mg/L constitutes the correct does of chlorine to be applied.\nIt is worth noting here that recommended residual chlorine level for drinking water is 0.5 mg/ litre, while for swimming pool sanitation it is 1.0 mg/ litre and for water bodies & post disaster it is 0.7 mg/litre', 'role': 'assistant'}]