I know that the answer to the question "__ saw the migration of the Han Chinese" is in "During the period of North–South division, Nanjing remained the capital of the Southern dynasties for more than two and a half centuries. During this time, Nanjing was the international hub of East Asia. Based on historical documents, the city had 280,000 registered households. Assuming an average Nanjing household had about 5.1 people at that time, the city had more than 1.4 million residents.". Can you tell me what it is?
The answer to this question is:
North–South division