Refer to the map below to answer the following question: Which of the following best explains why the population in some regions north of the Huang River stayed constant over this time period?

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