Refer to the map below to answer the following question:

A map image of ancient China is shown. The region is divided into sections. The map shows the population change over a period of time in China. In the northeast, the population has decreased. In the northwest, the population has increased slightly or remained the same. In central and south China, the population has increased considerably. In the southeast, the population reached the highest level of increase.

Which of the following best explains why the population in some regions north of the Huang River stayed constant over this time period?

A. Only a few new groups migrated into or out of the region so this allowed the population to stay constant.
B. Tang and Song farmers produced rich soil quality, which resulted in a population boom in some regions.
C. The number of invaders who settled in those regions about equaled the number of Song who fled.
D. The gains in population from an increased food supply about equaled the losses from constant warfare.