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 main constituent gas in CNG ? 
Select and explain. 
A. Acetylene, B. Butane, C. Ethane, D. Methane
D: Methane
Answer: D: Methane 
Explanation: None