Despite occasional stoppages from strikes or lockouts, the National Football League $($NFL$)$ is the undisputed leader in the professional American football industry. Recent attempts to launch competing leagues $($AFL$,$ USFL, UFL, WFL$,$ and XFL$)$ have been easily defeated or absorbed$,$ but the NFL is compelled to spend an enormous amount of money and effort innovating and marketing its entertainment services. This is an example of which of the Five Forces?

Threat of substitutes

Rivalry among existing firms

Bargaining power of supliers

Threat of new entrants