What Role Do Microbes Play in the Increase of Atmospheric

What role do microbes play in the increase of atmospheric carbon dioxide seen in this graph?

A) Microbial populations have been increasing since 1800,and this increase has led to increases in carbon dioxide levels.
B) Microbes convert other elements in carbon dioxide and have caused increases in carbon dioxide levels.
C) Microbial populations have been decreasing since 1800,and this decrease has led to increases in carbon dioxide levels.
D) Microbes fix carbon dioxide and help limit increases of carbon dioxide in the atmosphere.