TOEFL Listening: ETS-TOEFL听力机经 - 0U07UZC67W33M4F7P$

Why are seals and dolphins mentioned in the lecture? A. To give an example of animals that are disadvantaged by parasitic relationship B. To give an example of a symbiotic relationship where both organisms benefit C. To point out that all symbiotic relationships have advantages and disadvantages D. To compare the categories of commensalism and mutualism