What happened to the carbon that poisoned the atmosphere on earth?

I believe that the answer is that Carbon dioxide ( CO2 ) is still , building up in the atmosphere because we are still burning fossil fuels ( coal, oil and natural gas ) which emits CO2, and we are still cutting down trees which will not remove CO2 from the atmosphere. The natural carbon cycle is unable to remove all the extra CO2 , maybe only 60% of it , so the carbon is building up in the atmosphere and causing global warming .

Ibelive the answer is a