TOEFL Listening: ETS-TOEFL听力机经 - 732G5CFD71GH83E3P$

What does the professor suggest about the covering of sediment on the ocean floor? A. The amount that accumulates each passing year is highly predictable. B. Its thickness can be greater than his students might imagine. C. It is deepest in the areas closest to rocky or mountainous land D. It is responsible for the valleys found at the centers of oceanic ridges