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
Break point chlorination means: 
Select and explain. 
A. Start of chlorination process, B. End of chlorination process, C. When free residual chlorine starts appearing, D. After partial saturation of water with chlorine
C: When free residual chlorine starts appearing
Answer: C: When free residual chlorine starts appearing 
Explanation: The addition of chlorine to water to a point at which free residual chlorine begins to appear is called breakpoint chlorination.
The "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.
The point at which the chlorine demand of the water is met is called the "breakpoint". If further chlorine is added beyond the breakpoint, free chlorine begins to appear in the water.

Essentials of preventive and community dentistry
Soben Peter
5th edition
Page number 466