You Want to Know the Opinions of American School Teachers

You want to know the opinions of American school teachers about establishing a national test for high school graduation. You obtain a list of the members of the National Education Association (the largest teachers' union) and mail a questionnaire to 2500 teachers chosen at random from this list. In all, 1347 teachers return the questionnaire. The sampling frame is

A) the 1347 teachers who mail back the questionnaire.
B) the 2500 teachers to whom you mailed the questionnaire.
C) all members of the National Education Association.
D) all American school teachers.
E) all American school students.