A Gallup Poll Contacts People by Dialing Telephone Numbers at Random

A Gallup Poll contacts people by dialing telephone numbers at random. A possible source of nonsampling error in this poll is that

A) some people chosen for the sample refused to answer questions.
B) people without telephones could not be in the sample.
C) some people never answered the phone in several calls.
D) Both A and C are correct.
E) Answers A, B, and C are correct.