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
Residual chlorine in chlorination of water should be- 
Select and explain. 
A. 1 mg// after 1 hr, B. 0.5 mg/1 after 1 hr, C. 1 melt after 30 mins, D. 0.5 mg/1 after 30 mins
B: 0.5 mg/1 after 1 hr
Answer: B: 0.5 mg/1 after 1 hr 
Explanation: Ans. is 'b' i.e., 0.5 mg/I, afterl hr