Answer:
b. Agricultural advances, such as fast-ripening rice and the cultivation of more land, made food plentiful.
Step-by-step explanation:
The Tang and the Song dynasties era was an era of prosperity and innovation in which not only did China become the most advanced country in the world, but it also became the most populous, soaring to 100 million during the Song dynasty. One of the reasons for this dramatic population increase was China's advancements in farming, which improved the cultivation of rice thanks to a new variety of fast-ripening rice that began to be largely used in the country. This new variety allowed farmers to harvest two rice crops each year rather than one, and consequently, they produced more and more food that was sufficient to feed the rapidly expanding population.