Which of the following statements about the urbanization of early civilizations is true?

a.
early civilizations tended to take two main forms: nomadic tent dwellers and semi-nomadic herders, both of which existed in the desert and semiarid regions.
b.
early civilizations settled in coastal areas because oceanic trade and sea travel were important means of acquiring resources.
c.
most early civilizations were located in cooler climates, which supported bigger game hunting, herding, and gathering.
d.
early civilizations settled along river valleys because the proximity to water and food sources supported greater populations.