Refer to the Following Paragraph to Answer the Questions Below

Refer to the following paragraph to answer the questions below.
Robert Warner, president of a local TV station, recently conducted a study of TV viewers in his area. He obtained a list of all residential customers from the cable TV company billing list. Oddly enough, he put every customer name on a 3x5 note card, and mixed them up in one raffle barrel. He then choose 1000 note cards out of a possible 20,000 customer cards) . A trained interviewer visited each household and asked detailed questions about the viewing habits of various family members.
-What is the sampling frame of the study?

A) cable television viewers
B) the list of customers from the cable television company
C) all voters in the community
D) all households in the community