n Friday, S, a landowner, called B and offered him $500 to plow a field by Monday at 4:30. The offer explicitly stated that S wanted only a performance and did not want a promise, and you may assume that it was an offer for a unilateral contract. B immediately hopped on her tractor and set to work. On Saturday evening, after B had plowed half the field, S called and revoked his offer. B, furious, called her lawyer, who told her to go ahead and finish the job. B thought about it for a couple of days and finally decided that her lawyer was right. She finished the plowing Tuesday. As between S and B: