Which of the following transactions does not take place in a market for a factor of production?
a. Karl provides plumbing services for a plumbing company and receives an hourly wage from the company for his services.
b. Juanita works as a marriage counselor and her clients pay her on a per-hour basis for her services.
c. Trish owns several shopping malls and receives rent payments from the companies that operate those malls.
d. Ben sells advertising for a newspaper and receives a commission from the newspaper company for each advertisement that he sells.