TOEFL Listening: ETS-TOEFL听力机经 - Z58PF4P5Z5O2RSG5P$

According to the professor, what is one factor that makes categorizing symbiotic relationships difficult? A. New types of symbiotic relationships are always being discovered. B. It is not always clear what an animal gains or loses from a symbiotic relationship. C. The nature of the symbiotic relationship may change over time. D. Many symbiotic relationships are only temporary.