Answer the following question given this paragraph: 

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


Q: By breaking down organic matter, roundworms play an important role in which cycle?

 Choices:

- water cycle

- metamorphic cycle

- circadian cycle

- carbon cycle

A:
carbon cycle
