In the nationwide trial for the Salk polio vaccine (1954) some communities refused to participate in the random assignment of children to treatment or placebo. The investigators used an alternative method wherein they offered second graders vaccine and first and third graders acted as controls. Upon analysis, the investigators find that parents who volunteered their children for participation in the nonrandomized study were on average better educated, and earned higher incomes than the families that refused to participate. These children also tended to be absent from school more than non-participants. This may be a demonstration of

Group of answer choices

A. neyman bias

B. recall bias

C. non-response bias

D. selection bias

E. c and d