Hester asks Frank if he will do some yard work for $400. Frank says that he would like to think about it, and Hester says that is fine. The next day, Martha, Hester's friend, says that she will do the yard work for $350 because she needs some extra cash. Hester really likes Martha more than Frank and wants a cheaper price for the work, so she immediately accepts. Later that day Hester sees Frank walking up her driveway. She immediately opens the window and yells "I revoke the offer on the yard work" before Frank says anything. Frank replies that he accepts the offer and expects to be paid for the yard work. Do Hester and Frank have a valid contract, and why or why not? For how long is an offer open if no set expiration term is included in it?