This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Which one of the following method is used for the estimation of the chlorine demand of water - 
Select and explain. 
A. Chlorometer, B. Horrock's apparatus, C. Berkfeld filter, D. Double pot method
B: Horrock's apparatus
Answer: B: Horrock's apparatus 
Explanation: Horrock's apparatus is designed to find out the dose of bleaching powder required for disinfection of water. The indicator used is starch iodide solution. Though chlorometer is the instrument used to find the amount of chlorine in the given solution, chlorine demand is best identified by Horrock's. Park's Textbook of Preventive and Social Medicine, 25th edition, Page No. 788,776