Answer:
Invaders forced a huge portion of the population to the south.
Explanation:
During the era of Song Dynasty , Chinese citizens receive series of invasion that came from the Northern part of their country (Historian predicted that these invaders came from modern day' Kazakhstan's territory)
To avoid these invasions, many people at that time moved to the southern part of the country. This caused an increase in the amount of labor force in the southern region, which contribute to the significant growth of the southern region.
