Which of the Following Is True Regarding Voluntary Participation

Which of the following is true regarding voluntary participation?

A) No coercion can be used to obtain the research participants' informed consent.
B) It is still voluntary even if inmates perceive that they will receive better treatment from prison staff when they participate in a request by officials.
C) When a professor asks students to complete a survey in his or her class, those students never feel pressure to comply.
D) Specifically vulnerable to exploitation are the rich, the powerful, and the elderly.