This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Which one of the following is the level of residual free chlorine recommended for swimming pool water disinfection? 
Select and explain. 
A. 0.5 mg/litre, B. 0.8 mg/litre, C. 1 mg/litre, D. 5 mg/litre
C: 1 mg/litre
Answer: C: 1 mg/litre 
Explanation: Chlorination is the most widely used method of pool disinfection. Various workers have stated that continuous maintenance of 1mg/litre(1ppm) of free residual chlorine provides adequate protection against bacterial and viral agents. Park's Textbook of Preventive and Social Medicine, 25th edition, Page No. 788