TOEFL Listening: ETS-TOEFL听力机经 - S3D7A5TZ43W636G6P$

According to the professor, what is the source of color variations in hoodoos? A. Freeze-and-thaw cycles in the winter. B. Differences in the types of minerals in the rock layers. C. Differences in the amount of pressure exerted on the rocks. D. Periodic flooding from snowmelt and thunderstorm.