Why did the aztec city of tenochtitlán become one of the richest cities in the
world?
a. the richest residents of all the nearby cities came to settle in
tenochtitlán.
b. tenochtitlán became the center of the lucrative mesoamerican
gold-silk trade.
c. the aztecs discovered gold deposits just below the surface of
tenochtitlán's central lake.
d. the aztecs used tributes from conquered peoples to build
tenochtitlán into an impressive city.

D

Explanation:

A is incorrect because that's not how cities become rich.

B is incorrect because there was no Mesoamerican Gold-Silk trade.  (They didn't have silk!)

C is incorrect because it's just flat-out false.

D is correct because the Aztec Empire was made mostly of tributary villages (conquered peoples) that they collected wealth (tribute) from.  

he is known for discovering   the fossil of a female hominin australopithecine   known as "lucy" in the afar triangle region of hadar, ethiopia.

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