Buyline is an e-commerce Web site. It has come up with a promotional offer where buyers get a 60 percent discount on refrigerators if a minimum of 100 buyers agree to buy the product within 24 hours of the offer being announced. In this case, it is evident that BuyLine is a _____.

a.peer-to-peer e-commerce platform
b.social networking site
c.participatory e-commerce site
d.group buying platform