Which of the Following May Not Constitute Voluntary Participation

Which of the following may not constitute voluntary participation?

A) give students who complete the research questionnaire extra credit
B) allow students to fill out a questionnaire on their own time and when they finish put them in a box outside the departmental secretary's office
C) ask that students fill out a questionnaire and not include their name or student identification number
D) on in-class questionnaires have the professor leave the room while they are being filled out and turned in and allow students to remain anonymous