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.

What factors explain the major increase in population in half of China and a much smaller change in the other half?

an invasion from the north and rich farmland to the south
an invasion from the south and rich farmland to the north
overland trade and urbanization
seagoing trade and urbanization