Professor Mary Mancini Studied Class Level and Drinking Behaviour at Rocky

Professor Mary Mancini studied class level and drinking behaviour at Rocky Mountain University in Red Deer,Alberta.At Rocky Mountain,every student is required to live in one of six 20-story dormitories,and every student has a single room.All dormitory room phone numbers begin with the prefix 747 or 757 .She used the student telephone directory and,after a random start,selected every fifteenth student.She then mailed a questionnaire to the 1000 students selected and had two follow-up postcard reminders.A total of 900 students returned questionnaires.Later,Professor Mancini asked the university registrar's office whether any students were omitted from the directory. What is Mary Mancini's sampling frame?

A) The drinking behaviour of students
B) Students both in the directory and not in the directory
C) The student telephone directory
D) Rocky Mountain University
E) The class levels