How did radio companies such as nbc™, whose actual broadcasts were free, make money?

a. the media was a not-for-profit organization until the advent of television.
b. companies split and sold off those newly created companies for a profit.
c. broadcasters sold airtime to sponsors who then used it to run advertisements for their goods and services.
d. broadcasts were not, in fact, free. rather, listeners paid a subscriber's fee to hear the broadcasts.
e. broadcast companies received a cut of the profits from the sales of radio equipment.