We are going to talk today about the moon, our moon.First of all, the earth' s moon is unusual. Why? It' s larger than other moons or satellites in the solar system, in relation to its planet that is. Its diameter is more than a quarter that of the earth. And if you compare the earth and the moon in terms of substance, you find the moon isn't much like the earth. For example, the earth has a significant iron core, but the moon contains very little heavy materials like iron.That' s why its density is much lower than that of the earth. Now, one time it was believed that the moon and the earth were formed at the same time from the same material. But then wouldn't the moon have as much iron as the earth?Another theory is that the moon was formed elsewhere in the solar system, and then it was captured, so to speak, by earth. But study shows that the young earth would not have had enough gravitational force to stop a body the size of a moon from traveling through the solar system and pull it into orbit. The newest theory is called the big splash theory. Here, the new young earth was hit by another big planet. Most of the colliding planet entered the earth and became part of it. But the huge impact created a vapor that shed out into space and eventually condensed as the moon. Because this material came mostly from the earth surface crust, not the iron core, the moon contains almost no iron. Well, as plausible (似是而非) as it sounds, it's only a theory, and we can't be sure that this is what really happened, that this is how the moon originated. Plenty of research remained to be done.
解析
What does the Prof. mainly discuss?
主旨题。根据第一句话和通篇内容可知，本演讲主要讨论的是月球起源。因此，正确答案为D。