A large concert promoter that operates several hundred concert locations around the country wants to survey the managers at these locations to ask their opinions about how to improve attendance at concerts. Because the survey is rather lengthy, the promoter does not want to ask all the managers and decides to ask a random sample of managers instead. The promoter organizes the concert locations into 20 different geographic zones, randomly selects 5 zones, and surveys all of the managers in those 5 zones. Is this an example of stratified random sampling, systematic sampling, or cluster sampling?