What is the first example of radio advertising?

a.
The Du Pont sponsored history program, "Cavalcade of America," broadcast on the Columbia Broadcasting System

b.
"Dr. John Romulus Brinkley's Lectures on Vigor and Vitality," a program featuring patent medicine

c.
AT&T's "toll broadcasting" station, WEAF New York

d.
RCA's use of sponsors on its National Broadcasting Company