TOEFL Listening: ETS-TOEFL听力机经 - 3B6XL439MQMM354MP$

Why does the professor mention worms and fungi? A. To emphasize the interdependence of plants and other organisms. B. To emphasize the importance of oxygen in the atmosphere. C. To point out factors that slowed the spread of plants on land. D. To give examples of organisms that helped form topsoil.