On Monday, Xi tells Yard Work Inc., that he will pay $500 if a variety of tasks are completed by Friday. On Wednesday, when Yard Work is more than half done with the work, Xi says that he has changed his mind. These parties had

a.an expired contract when Xi said that he had changed his mind.
b.a unilateral contract as soon as Yard Work began to perform.
c.no contract.
d.a quasi contract when Xi said that he would pay for certain work.