【单选题】听力原文: The origin of Earth's moon, the largest moon in the solar system, is still something of a mystery. There are some theories about its origin however. Now, keep in mind that a theory of the moon's...
A.
Differences between the planets in the solar system.
B.
Methods of determining the composition of Earth and the Moon.
C.
Why Earth has an iron core.
D.
The origin of the Moon.