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
Horrock’s apparatus estimates 
Select and explain. 
A. Free chlorine, B. Chlorine demand, C. Both (a) + (b), D. Combined chlorine
B: Chlorine demand
Answer: B: Chlorine demand 
Explanation: Used To find out the dose of bleaching powder required for disinfection of water, i.e. ‘Chlorine demand estimation of water.'