Given the task definition and input, reply with output. In this task, you are given two facts, and a multiple-choice question. Based on the given facts, answer the question with index of the correct option (e.g, "A").

Fact1: Roundworms make up the phylum Nematoda., Fact2: Also, nematoda and rotifera are important groups contributing to soils., Question: Where are roundworms useful in an ecosystem? (A) killing animals (B) soils (C) fields (D) bacteria (E) eating plants (F) wetland (G) food (H) alveoli
B