The key to solve this problem is to understand the meaning of quotation. Xin Qiji thinks that northern China is a self-sufficient natural economy, and the gap between the rich and the poor is not big. Therefore, we can't draw the conclusion that agriculture in the north is more developed than that in south China, and exclude A. In the south, "many farmers get sick in the end" and "merger is harmful to prosperity", which is the reason for the gap between the rich and the poor, but it is not the root cause. Excluding B, it can't be seen from the material that the north is more stable than the south wind. "No" refers to industry and commerce, and the south is "more at the end", which shows that Shanghai in the south is economically developed, so the answer is C.