What motivated and
sustained of the silk roads sea roads, and the
sand roadsmod –

i think the answer is c, rivers.

explanation:

sometimes, borders fall along natural boundaries like rivers or mountain ranges. for example, the boundary between france and spain follows the crest of the pyrenees mountains. for part of its length, the boundary between the united states and mexico follows a river called the rio grande. the borders of four countries divide africa’s lake chad: niger, chad, cameroon, and nigeria.

- "made the south's economy grow rapidly and a small effect that a lot of people don't know is that it made the more demand of slaves and cotton products. and also new states formed due to plantations growing in size."

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