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 MOST ourable pH for chlorine to act as a disinfectant? 
Select and explain. 
A. 6.5, B. 7, C. 7.5, D. 8
B: 7
Answer: B: 7 
Explanation: Disinfecting action of chlorine is mainly due to hypochlorous acid and to a small extent due to hypochlorite ions. Hypochlorous acid is 70 to 80 times more effective than hypochlorite ions. Chlorine acts best at a pH of around 7 because of the predominance of hypochlorous acid. When pH exceeds 8.5, chlorine is unreliable as a disinfectant. Ref: Park 21st edition, page: 661.