TOEFL Listening: ETS-TOEFL听力机经 - 14SU369W4PE8P11EP$

What points does the professor emphasize about soil samples collected during the Apollo space missions? A. They are less abrasive than simulated lunar soil. B. Some of their properties may have changed. C. Each sample was taken from a different layer of the regolith. D. There are very few of them left for research.