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
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. 
Select and explain. 
A. Break point., B. Chlorine demand., C. Loss of head., D. Back wash.
B: Chlorine demand.
Answer: B: Chlorine demand. 
Explanation: None