TOEFL Listening: ETS-TOEFL听力机经 - FO4UF5565H21Q3_4P$

Why does the professor mention potholes? A. To emphasize that frost wedging has a profound effect on rock. B. To show that hoodoos are surprisingly fragile. C. To point out that potholes are not very common in deserts. D. To show that the amount of acid rain in Bryce Canyon is increasing.