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: Unknown
Which of the following is non-natural gas causing greenhouse effect - 
Select and explain. 
A. CO2, B. H2O, C. Hydrofluorocarbons, D. Ozone
C: Hydrofluorocarbons
Answer: C: Hydrofluorocarbons 
Explanation: Ans. is 'c' Le., Hydrofluorocarbons Greenhouse effect o The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in the atmosphere warm a planet's lower atmosphere and surface. Greenhouse gases include - Natural 1) CO2 3) Methane 5) Ozone 2) Nitrous oxide 4) Water vapor Non-natural gases include - 1) Hydrofluorocarbons 3) Perfluorocarbons 2) Sulfur hexafluoride