North Korea and South Korea are separated by .

a brick wall
the Choson River
the Seoul Mountains
a demilitarized zone

By A demilitrarized zone

Explanation:

In 1950, after years of mutual hostilities, North Korea invaded South Korea in attempt to unify the peninsula under its communist rule. The subsequent Korean War, which lasted from 1950 to 1953, ended with a stalemate and has left the two Koreas separated by the Korean Demilitarized Zone (DMZ) up to the present day.

the mongolian invasion of china and central asia resulted in a disruption of the silk road as the chinese garrisons lost support from the chinese kingdoms which fell under the mongols. however, as the mongols established a formal dynasty in china, the successors of genghis khan restored the garrisons along the silk road allowing the safe travel caravans again.