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
Level of residual chlorine after one hour 
Select and explain. 
A. l mg/L, B. 1.5 mg/L, C. 0.5 mg/L, D. 2 mg/L
C: 0.5 mg/L
Answer: C: 0.5 mg/L 
Explanation: None