Question

if the number of concerts held in a city is correlated to the number of local radio channels, what third variable might most likely be responsible for this correlation

Answer 1

From my research, the question had the following choices:

the number of songs played on each channel     
the weather conditions of the city
the number of music lovers in the city
the duration of each concert

Among the choices, the correct answer is "the number of music lovers in the city". The more concerts there are in a city corresponds to the number of music lovers. Radio channels usually play music, so having a lot radio channels in a city must mean that there is a large audience.