Answer:
By using Gateway Arch as the symbol to represent United States expansion policy.
Step-by-step explanation:
Gateway Arch, which is located in St. Louis Missouri, was designed by Error Saarinen in 1948 and its construction was completed in 1965, costing a whopping is $13million at the time.
It derived its name from "Gateway to west", a slogan used by the US during the westward expansion in the 19th century.
The artist used the Gateway Arch symbol to represent the a door to the western part of United States.