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
Chlorine demand of water determined by: 
Select and explain. 
A. Horrocks apparatus, B. Chloronometre, C. Berkefield filter, D. Double Pot
A: Horrocks apparatus
Answer: A: Horrocks apparatus 
Explanation: Horrocks apparatus