TOEFL Listening: TPO-TOEFL阅读TPO - C7O00E8EI49LFT_0N$

In stating that "no asteroid itself has ever been recovered", the author emphasizes which of the following? A. The importance of the indirect evidence for a large asteroid B. The fact that no evidence supports the asteroid-impact hypothesis C. The reason many researchers reject the Alvarez hypothesis D. The responsibility of scientists for not making the effort to discover the asteroid itself