Which key sentence from the passage identifies this
text as a primary source?

Cultural convergence is the theory that two cultures will be more and more like each other a very notable example of this is around the u.s.-mexico border, where 

Humans have survived and thrived in mexico city by using technology to create greater access to land. when the spanish conquered the aztec capital, tenochtitlan, in the 16th century, they built mexico city by draining the lakes and filling in the lake water to create more arable land for agriculture. this technique of land reclamation has continued to be fundamental to human civilization in mexico city. 

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