In order to determine how American undergraduate college students feel about eliminating spring break in order to finish spring term a week early, a survey was conducted. 200 undergraduate students from the University of Miami (FL) were interviewed. Both of the interviewers hired to conduct the survey were told to interview 25 freshmen, 25 sophomores, 25 juniors, and 25 seniors. Of the 200 students interviewed, 20% were in favor of the elimination of spring break, 70% were opposed, and 10% had no opinion. The results of this survey are unreliable primarily because of

• the absence of a control group.
• selection bias only.
• both selection bias and non-response bias.
• nonresponse bias only.
• None of the above