Mountains were important to early Greek city-states because they: A.
stopped the city-states from becoming seaports.

B.
provided the city-states with good farmland.

C.
helped the city-states capture more territory.

D.
kept the city-states separate from one another

D.  kept the city-states separate from one another

Explanation:

It is said that the "mountains" played an important role to the early Greek civilization. It kept the city-states or regions separate from one another. Every city state was originally ruled by a king; however, over the course of years, different government forms emerged. The two most popular city-states were Sparta and Athens. Athens incorporated direct democracy into their political system. This allowed all the males in the region to have a right in the making of the laws.