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
Which of the following is the first choice for use in disinfection of large bodies of water? 
Select and explain. 
A. Chlorine gas, B. Chloramine, C. Perchloron, D. Bleaching powder
A: Chlorine gas
Answer: A: Chlorine gas 
Explanation: For disinfecting large bodies of water, chlorine is applied as chlorine gas, perchloron or chloramine. Among these, chlorine gas is the first choice as it is cheap, quick in action and easy to apply. Ref: Park 21st edition page: 661.