While it is true that effects of disease have been overlooked, the ancient kingdom of People X began to collapse because of the violence, not the new diseases brought by explorers. This much is evident from the fact that the kingdom first fell in the south, in the region under attack by explorers, and only later in the northern part of the kingdom.
Which of the following, if true, would most strengthen the argument above?
【选项】
The northern portion of the kingdom was not substantially weakened until after the explorers attacked that region.
选项是否正确？