In Which of the Following Is the Contract Not Ended

In which of the following is the contract not ended by agreement?

A) Goode contracted to sell his pony to Reston. Goode then changed his mind and told Reston he didn't want to sell the pony but would give him two lambs instead. Reston agreed to the substitutes.
B) Randy visited Terry two weeks after the contract but one month before performance date to say he wouldn't do it. Terry had already paid Randy for the work. After hearing Randy's story, Terry said, "Just forget it, good bye and good luck."
C) Sean hired two students to do a market survey for him for $3,000. Before the students had begun the work, they both had received offers for full time jobs. They told Sean they wanted out of the contract. Sean said, "O.K., I'm short of cash anyway."
D) After party A of the contract paid, party B said he couldn't perform. Party A agreed to waive B's obligation if B would return the money paid to him. B agreed.
E) The event contained in the condition subsequent provision of the contract has happened.