TOEFL Listening: TPO-TOEFL阅读TPO - 310P2N52W436FYL0N$

All of the following questions are answered in paragraph 2 EXCEPT: A. Why was Callisto frozen solid before differentiation was complete? B. What allows the process of differentiation to get started? C. Why is it easier for an icy body to differentiate than a rocky one? D. How do astronomers know that Callisto lacks a dense core?