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
Horrok's apparatus determines Chloride which has to have a holding level of - 
Select and explain. 
A. 0.1 mg/L, B. 1 mg/L, C. 0.01 mg, D. 0.5 mg/L
D: 0.5 mg/L
Answer: D: 0.5 mg/L 
Explanation: Ans. is 'd' i.e., 0.5 mg/L Horrock's apparatus is used to find out the dose of bleaching powder required for disinfection of water by chlorination. o The principle in chlorination is to ensure a free residual chlorine o10.5 mg/litre at the end of one hour contact.