What did the 18th century Americans learn from ancient Greece and Rome?

If by 18th century Americans you mean the framers of the constitution than they learned quite a bit. The idea of a representative republic came directly from the Romans. The idea of a republic where the government's actions are restrained by a written rule of law, the constitution for us and the twelve tables for Rome. However, they also learned that democratic republics, like Rome's, can crumble from the non-participation, (or over-participation like a mobocracy) of the people and the corruption of the government. This is why they came up with the system of checks and balances and the separation of powers.

Explanation:

because mexico wanted to choose a centralized location

d. china

good luck

d is the best choice