Why Must Researchers Be Mindful of Subject Payments to Maintain

Why must researchers be mindful of subject payments to maintain their participation in a longitudinal study?

A) Researchers usually do not have enough money to last the entire study because they need to pay for multiple sessions and most researchers run out of funds.
B) Participants only participate in paid research.
C) There are ethical concerns about voluntary participation if researchers pay too much to retain participants.
D) Researchers need to be ready to bargain a price with each participant.