Answer:
The tread on the road declined sharply till in the 13th century,when the conquests of the Mongols ushered in an era of frequent and extended contacts between East and West...
With less cost,harassment and danger,many goods and materials that the Silk Road could not transfer were conveyed through the sea route.
1) States have powers and their own governments yet come together under one federal power to form the United States of America.
2) Checks and balances- the executive checks the legislative, the legislative the judicial, etc. For example, the president can veto Congressional legislation, and the Supreme Court can declare laws and presidential actions unconstitutional. In the same way, Congress can impeach a president.
3) Amendments! There are currently 27. The first 10 are known as the bill of rights.
