Which of these events deserve to be called watershed events in american history? because?

1) the invention of basketball in 1896.
2) the discovery of penicillin in 1928
3) the election of barack obama to the presidency in 2008.
4) prohibition of alcohol in the 1920s.
5) brown v. the board of education supreme court case.
6) the invention of the cell phone.
7) the battle of the alamo.
8) rap music.