Robert Rabbitear, president of a local TV station, recently conducted a study of TV watchers in his area. He obtained a list of all residential customers from the cable TV company billing list. He selected random customers from the 20,000 households on the billing list by taking every 40th household. A trained survey interviewer then visited each of the 500 households and asked detailed questions about the viewing habits of various family members.What is the sampling frame of the study?