Why Is There a Special Concern with Anonymity and Confidentiality

Why is there a special concern with anonymity and confidentiality when a researcher studies "captive" populations like prisoners?

A) Gatekeepers,or those in positions of authority,may restrict access unless they receive information on subjects.
B) Pseudonyms or false names cannot be used for prisoners.
C) Researchers are legally obligated to tell the police about what they have learned from the subjects.
D) All of the above are reasons for special concern.
E) None of the above are reasons for special concern.