Consider the following list, and classify each item according to the appropriate type of two-sided market-audience-making, matchmaking, shared- input, or transaction-based-and write a one-sentence answer justifying your classification. (See pages 610-612. Hint: You may wish to check out the firms' Web sites to assist in answering this question.)

a. Mastercard.com

b. FreeBSD.com

c. Plentyofish.com

d. WSJ.com