Which of the Following Reasons Is at Least in Part

Which of the following reasons is at least in part responsible for the peopling of the Yangzi region in the southern part of China during the Ming era?

A) The introduction of crops from the Americas that could be cultivated on inferior soils that did not require irrigation
B) The enforced migration of remaining Mongols within the Chinese population to the region of the Yangze
C) The abandonment of rice and millet cultivation in the region in favor of wheat introduced by Portuguese merchants
D) The opening of the region to settlement from Siam and Vietnam