Answer:
An agonist binds to the receptor and produces an effect within the cell. An antagonist may bind to the same receptor, but does not produce a response, instead it blocks that receptor to a natural agonist.
The Great Dust Bowl
Ku Klux Klan
Prejudice
Prohibition
Rivers played a major role in the settlement of the West of the US in that places that contained rivers were much more likely to be settled--due to the fact that it was easier to transport both goods and humans.