Studies have shown that the number of bars and the number of churches in 20 different cities are highly and positively correlated. Which of the following explanations seems most appropriate? 

a. The presence of more churches causes an increase in alcohol consumption

b. The presence of more bars causes an increase in church attendance.

c. The number of bars and the number of churches are both related to a third factor—population size—and so are correlated to one another.