Final answer:
The expansion of trade networks was caused by the rise of empires like the Mongol Empire and new transportation technology.
Step-by-step explanation:
The expansion of trade networks was caused by the rise of empires like the Mongol Empire and new transportation technology. As empires grew and expanded their territories, they needed efficient ways to transport goods, people, and ideas. This led to the development of new transportation technologies such as improved ships, roads, and caravans, which facilitated the expansion of trade networks across regions.