The most accurate response regarding how Greek and Roman democracies impacted the design of the U.S. government in the eighteenth century is:
The designers of the government borrowed elements of these governments in structuring the U.S. federal government.
This reflects how the framers of the U.S. Constitution were influenced by concepts such as republicanism, civic virtue, and the importance of a system of checks and balances, which can be traced back to ancient democracies and their political philosophies.