Consider a keyword auction with 4 bidders, b1, b2, b3 and b4. The amounts each bidder is willing to pay per click are: b1: 10/click b2: 8/click b3: 12/click b4: 7/click There are 3 positions to be awarded. The click frequencies are: 1st position: 400 clicks per week 2nd position: 200 clicks per week 3rd position: 100 clicks per week A bidder's valuation is her willingness-to-pay per click times the expected number of clicks per week. (a) What is the assignment under a VCG auction? (b) What is the total amount each bidder pays in the auction? (c) Interpret payments based on the idea of opportunity cost.