What Criterion Must Be Met to Include Prisoners as Subjects

What criterion must be met to include prisoners as subjects in a research study?

A) The research must have direct relevance to the prison population.
B) The prisoner must receive compensation in some form for his or her participation.
C) At least two witnesses are required to demonstrate that the prisoner was not coerced to participate.
D) Prisoners selected as participants must either have life sentences or death sentences.