Which of the following statement most accurately characterizes the development of professional, commercial radio during the 1920's?

a) The first national network, National Public Radio, was created with federal subsidies in 1928.
b) The federal government licensed radio stations, but their revenue came primarily from advertisers and corporate sponsors.
c) One of the most popular radio shows, Amos "n" Andy, was notable for breaking the color line and allowing African american actors to perform.
d) Professional radio broadcasting began in 1924, when a Pittsburgh station covered the Democratic and Republican national conventions.