Explanation

Reading the question: we read about the ancient kingdom of People X. The conclusion here is that violence, not disease, brought them down. The evidence concerns the order in which the parts of the country collapsed. To strengthen this argument, we'll want to strengthen the logical connection between the geographical evidence and the attribution of violence over disease. For example, maybe disease also struck first in the part that fell first to violence? That would undermine the connection. We look for something relevant to this geography-attribution question.

Applying the filter: choice (D) is pretty close to the specific prediction we had in mind. The other answer choices are tempting. Choices (C) and (E) both give comparisons that are not material to whether violence or disease initiated the problems. We focus on that phrase in the prompt--began the attack. Choices (A) and (B) are talking about beginning the attack, so they are both out.

Logical proof: we can analyze choice (D) by extreme cases. If the regions of the country were equally susceptible to disease, then it would be less likely that one part would fall first, as was the case. That would, indeed, strengthen the explanation that violence caused the downfall in the south. Consider the contrary: if, say, the south was much more susceptible to the diseases, then the south could have fallen first, due to disease, not violence, and the presence of violence would be a coincidence. So (D) strengthens the argument. The correct answer is (D).