What led to the population increase in china during the tang and song dynasties?

Explanation:

The population of China doubled in size during the 9th, 10th and 11th centuries. This growth was made possible by expanded rice cultivation in central and southern Song, the use of early-ripening rice from south-east and southern Asia, and the production of widespread food surpluses.

the conditions that made the byzantine empire vulnerable to attacks by the muslin turks were the sack of constantinople by western crusaders, the conquests of the sultanate of rum, presence of rival kingdoms and finally the set up of fiefdoms at the empire which were owned by the ghazis (turkish warriors).