Why Does the Text Say There Is a Greater Density

Why does the text say there is a greater density and cultural diversity of Native Americans living in California,than in the rest of North America,prior to European contact?

A) A greater number of tribal conflicts caused groups to split and become isolated.
B) The mild climate and rich diversity of resources.
C) Past periods of disease forced isolation between many groups that led to cultural uniqueness.
D) The flat terrain and lack of wetlands in the Central Valley drew many Native American immigrants from the surrounding Aztec,Mayan,Great Basin and Puget Sound tribes.