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Hey, i was what is consisted in a sustainable city? like what unique designs they have, etc. for answering ; )

Acivilization exists for several hundred yeaars, developing its own language. such a civilization probably has been: a. farming corn b. suffering from a lack of contact with the outside world. c. isolated by mountains or some other physical barriers. d. starving from lack of trade.

What is the thickest layer of earth called?