TOEFL Listening: ETS-TOEFL听力机经 - A1Y388J5OT5D3M13P$

Why does the professor discuss the name of the Cryogenian period? A. To provide backgrounds for the basis of the Snowball Earth hypothesis B. To suggest that the Snowball Earth hypothesis is inappropriately named C. To contrast the Cryogenian period with later geologic periods D. To point out that the Cryogenian ice ages started in several different ways