Unified government is the lack of checks and balances between the presidency and Congress.
no guarantee that the president will get legislation passed.
a myth.
essential to the preservation and maintenance of American democracy.
None of these are correct.

explanation:

the addition to geographic boundaries that might have  kept the people of the huang river civilization from traveling to far distance region is that they had learned about the other river valley civilizations but feared contact would bring invasion.

hope this .

plz give me brainliest!