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': '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 water. \nSelect and explain. \nA. Break point., B. Chlorine demand., C. Loss of head., D. Back wash.', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]