Betty Buyer wishes to enclose her back yard with a fence. She reads reviews online and decides that Sam Carpenter is the best choice to construct the fence. On Monday, she leaves Sam a detailed message that includes the address, the exact type of fence she desires, the time for completing the project, a promise she will pay $600 for the job, and the fact that Sam can either call her back and accept or simply show up and begin work. On Tuesday, without speaking to Betty, Sam shows up at her house and begins to build the fence. When he's more than halfway through, however, Betty runs into her yard and yells "I revoke!" and orders Sam off her property. Betty:
(A) Has validly revoked her offer because Sam never validly accepted it by completing performance.
(B) Never made an effective offer.
(C) Cannot revoke her offer because Sam has begun to perform.
(D) May validly revoke, and Sam may also discontinue performing without consequence if he wishes to do so.