AU Industries is in the process of selling shares of stock in an auction IPO. At the end of the bidding period, Luther's investment bank has received the following bids: (1) $10 - 250,000 shares; (2) $15 - 450,000 shares; (3) $20 - 500,000 shares; (4) $25 - 300,000 shares. What will be offer price of these shares if AU is selling 1 million shares?

a.$22.50

b.$15.00

c.$17.50

d.$20.00