Answer:
A. Greenhouse gases levels would decrease
Step-by-step explanation:
Burning fossil fuels like coal, oil and natural gas for energy releases greenhouse gases like carbon dioxide and methane into the atmosphere. These greenhouse gases contribute to global warming by trapping heat in the atmosphere. If humans stopped burning fossil fuels, it would dramatically reduce the amount of new greenhouse gases being emitted. This would allow atmospheric greenhouse gas levels to gradually decrease over time as natural processes absorbed some of the excess gases.
The other answers are not likely results:
B. The polar ice caps would melt more quickly - Stopping fossil fuel combustion would actually help slow the melting of polar ice caps by reducing global warming.
C. There would be less oxygen in the atmosphere - Fossil fuel combustion does not significantly impact atmospheric oxygen levels.
D. Global warming would increase - Ending fossil fuel use would lead to less global warming in the long run by reducing greenhouse gas emissions.
So in summary, the most immediate and likely result of humans stopping fossil fuel combustion would be decreasing levels of greenhouse gases in the atmosphere, as option A states. This would then help slow climate change impacts like polar ice melting.