103. Read the following passage, then answer the question below.

At its height in the thirteenth century, the Mongol Empire stretched from Korea to Hungary, the largest contiguous land empire in history. Governing such a territory posed a problem that conquest alone could not solve: how could a ruler learn what was happening, and send orders, across thousands of kilometers? The Mongol answer was the yam, a relay network of postal stations established across the empire.

Stations were placed at regular intervals, each stocked with fresh horses, food, and lodging. A courier carrying official dispatches would gallop to the next station, exchange his tired mount for a rested one, and continue, sometimes wearing bells on his belt to warn the next station to prepare. Under good conditions, couriers could cover more than two hundred kilometers in a day, far faster than any ordinary traveler. Authorized travelers, including officials, envoys, and merchants with official sanction, carried a metal tablet called a paiza that entitled them to supplies along the route.

The system was costly, and the burden fell heavily on local populations, who were required to provide horses and provisions. Abuses were common: some officials used the yam for private purposes, and complaints from peasants reached the court. Rulers tried to limit the damage with inspections and punishments, with mixed success. Nevertheless, the yam did more than carry messages. Because the stations hosted travelers, they encouraged trade and the movement of ideas, and the security the Mongols provided along major routes helped merchants and missionaries travel widely. Marco Polo, who described the system with admiration in the late thirteenth century, was only one of many foreigners who benefited. When the empire fragmented, the network deteriorated, but its design influenced the postal systems of later states.

The word 'its' in the last sentence of the passage refers to