TOEFL Listening: ETS-TOEFL听力机经 - 9ED48U8L1V71914WP$

Why does the professor say this: A. He believes he has accidentally treated some students in the class badly. B. He recognizes that he has not yet introduced a key concept. C. He realizes his question did not have any correct answer. D. He wants to point out the complexity of the topic of the lecture.