Which is not an example of a seller offering a signal of quality?
a. Chau offers customers a discount if they write reviews of
her manicures on Yelp.
b. Hui Er proofreads her job application and cover letter four times to make sure it shows that she is careful about details.
c. Mak offers a money-back- guarantee on his product.
d. Ian's Auto Body Shop offers a warranty on its work.