Final answer:
The sizable population rise in Song China during the eleventh century was caused by the introduction of new farming techniques, urbanization and commercialization, and migration from neighboring regions.
Step-by-step explanation:
The sizable population rise in Song China during the eleventh century was caused by a combination of factors. One important factor was the introduction of new farming techniques. Peasants in the Song dynasty began planting new strains of rice that ripened quickly enough to yield two harvests per year, increasing the food supply.
Additionally, urbanization and commercialization played a role in the population rise. As the economy developed internally, there was an increase in urbanization and the availability of non-agricultural jobs. This attracted many rural inhabitants to move into cities.
Migration from neighboring regions also contributed to the population increase. Refugees from Chinggis Khan's conquests sought more hospitable conditions and migrated to Song China. This influx of people further boosted the population.