The correct answer is A. I, II, and III only.
Here's the evaluation of each statement:
I. It is a natural process - True. The greenhouse effect is a natural process that warms the Earth's surface.
II. It is difficult to get fossil fuel - True. Extracting fossil fuels can be challenging due to environmental, geological, and technological factors, although this statement may be subjective.
III. It is caused by gases like chlorofluorocarbons - True. While chlorofluorocarbons (CFCs) are not the primary greenhouse gases (CO2, CH4, and N2O are), they do contribute to the greenhouse effect and ozone depletion.
IV. Some human activities enhance the greenhouse effect - True. Human activities, such as burning fossil fuels, deforestation, and industrial processes, increase the concentration of greenhouse gases in the atmosphere, thereby enhancing the greenhouse effect.
However, if we analyze the options, the best matching answer, considering the context, would actually be C. I, II, III, and IV only because while I, II, and III are more straightforward assertions, IV is also true in the context of human activities affecting the greenhouse effect.
Therefore, the most appropriate selection would be C for comprehensiveness, acknowledging that all statements provided are accurate.