TOEFL Listening: TPO-TOEFL阅读TPO - 4DD7Z31V472SR32ON$

What is the main purpose of paragraph 3? A. To describe the physical evidence collected by Hsu and Ryan B. To explain why some of the questions posed earlier in the passage could not be answered by the findings of the Glomar Challenger C. To evaluate techniques used by Hsu and Ryan to explore the sea floor D. To describe the most difficult problems faced by the Glomar Challenger expedition