Hi @kyland, while A is fairly inclusive, it completely excludes the third paragraph, and only tangentially considers the uncertainty within the scientific community about what the LHB was. It really only serves to define what the LHB was, but even in that definition it on its key facts including the uncertainty about its true nature, and the fact that impacted the earth as well.
While E is more focused on the third paragraph, it does speak to the main point of the passage. The passage lays out a phenomenon, explains three points of view about that the phenomenon was, and then provides a new piece of evidence that supports one of the three explanations. The first two paragraphs are mostly background, while the final paragraph is introducing new information which is critical to our understanding. It is ok for a main point not to be a summary (and indeed it should not just be a summary when the passage leans towards one view) but rather it should capture the most crucial aspect of the passage. In this case, the new evidence is the most important part of the passage.
