Why was ancient greece divided into a collection of city-states?

Correct answer choice is :

When the Greek nomadic clans occupied land to settle on and become agricultural, the tribes took pieces of river plains surrounded by mountains, building a castle and then a city as the center of the tribal patch. A city protected its region from others, and so the independent city-state became the foundation of the Greek world. When the cities sent out their growing excess population which could not be supported a city's limited land, they, in turn, took a patch of land around the Mediterranean and more cities grew up, ultimately numbering a pair of thousand.

i believe it d) heterogeneous

what class is this for i can then