A is correct about the relative size of the companies that built the industry to the major players nowadays, but I thought I should add that the only place you find clusters of stations commonly owned by the big companies dominating are in the top-50 markets. Some companies, like Cumulus and Univision, do operate clusters not only in the majors but also in markets below #51 (Albuquerque comes to mind) but for the most part you're going to find local or regional companies as the multiple-station owners.
In small and unrated markets, you're still going to find standalone stations owned by local interests.