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
What is the reagent used in Horrock's apparatus for determining chlorine demand of water – 
Select and explain. 
A. Orthotoluidine, B. Methylene blue, C. Starch iodide indicator, D. Sodium bicarbonate
C: Starch iodide indicator
Answer: C: Starch iodide indicator 
Explanation: None