TOEFL Listening: ETS-TOEFL听力机经 - G_CLS84HK0FQ56U7P$

Why does the professor say this: A. To signal that he is about to offer a contrasting way to consider bacteria. B. To imply that some types of bacteria have a more complicated structure than others. C. To encourage her students to point out interesting facts about bacteria to the class. D. To indicate that he agrees with the studens' opinion about bacteria.