Which of the following is a sink for $\text{CO}$?

Carbon monoxide ($\text{CO}$) is a highly toxic gas. Natural sinks for $\text{CO}$ include certain micro-organisms present in the soil that can oxidize it to ${\text{CO}}_2$ or convert it into other organic matter. While haemoglobin binds with $\text{CO}$ to form carboxyhaemoglobin, it is not considered a natural environmental "sink" in the context of reducing atmospheric $\text{CO}$ levels.