Fact 1: Roundworms may be free-living or parasitic. Fact 2: Parasites are organisms that live off a host. Given the two facts above, aside from free-living, where else can roundworms exist?

Pick your answer from:
[A]. mouth of rivers;
[B]. in the sky;
[C]. loose soil;
[D]. our planet;
[E]. Earth orbiting the Sun;
[F]. live off a host;
[G]. molecules;
[H]. h2o;
The answer is:
[F].