Answer the following question given this paragraph: 

Roundworms may be free-living or parasitic. Free-living worms are found mainly in freshwater habitats. Some live in soil. They generally feed on bacteria, fungi, protozoans, or decaying organic matter. By breaking down organic matter, they play an important role in the carbon cycle.


Q: Roundworms feed on various things, one of them being decaying organic matter.  doing this, they play an important role in what famous cycle?

 Choices:

- life cycle

- soul cycle

- water cycle

- carbon cycle

A:
carbon cycle
