The Olmecs were descendants from

Europe
Africa
Asia
North America

Africa

Explanation:

While historians have speculated that the facial features of some monumental carved heads indicate an African origin of these people, most scholars believe that the Olmec, like other native Americans, descended from Asian ancestors who entered North America during the Great Ice Age

crop rotation kepy different plants in the soil instead of once constintly. when switching out plants you have different nutrients being absorbed at different speeds and different roots growing differently therfore the spoil and water flow have to adapt and carry fresh nutrients to carry the constant changes that were going on in the soil.

c, they united with once