Refer to the Following Paragraph to Answer the Questions Below

Refer to the following paragraph to answer the questions below.
Professor Alco Drunkman studied class level and drinking behavior at Wild State University in Boozville, Texas. Every student is required to live in one of six twenty-story-tall dormitories, and has a single room. All dormitory room phone numbers began with the prefix 747 or 757 (e.g., phone numbers are 747-0001 to 757-9999) . He used the student telephone directory, and after a random start selected every 15th student. He then mailed a questionnaire to the 1,000 students selected and had two follow-up postcard reminders. A total of 900 students returned questionnaires. Later, he asked the university registrar whether any students were omitted from the directory.
-What is Drunkman's sampling frame?

A) the drinking behavior of students
B) students both in the directory and not in the directory
C) the 2002 student telephone directory
D) Wild State University
E) the class levels