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.

a social contract.

"a social contract is an agreement between citizens and their leader where the citizens give up certain freedoms in exchange for protection. the constitution of the united states is an example of a social contract."

the main reason that the constitution did not proclaim all men to be free and equal is because of the issue of slavery.