As part of an advertising campaign, a plane flew over the beach in Ocean City trailing a banner that read: "Boardwalk Bistro will give $100 to the first person to swim from the inlet at 1st Street to Bistro's at 33rd street." Daisy was a very strong swimmer and accepted the challenge. When she got to 25th Street, however, the plane appeared again with a banner that read: "Bistro withdraws its offer." Bistro's first banner had contained an offer to form what type of contract?