Which of the Following Represents a Potentially Coercive Incentive for Participating

Which of the following represents a potentially coercive incentive for participating in research?

A) Covering the cost of transportation to the research site.
B) Providing a small stipend to compensate research subjects for their time.
C) Paying $25 per hour for research participation to individuals who usually make less than minimum wage.
D) Providing a $5 gift certificate to employed individuals who can easily afford to purchase a $5 item whether or not they participate in the research study.