Roberta offers to sell certain land to Emily for $10,000. This offer is made by a letter, signed by Roberta and mailed on March 1, 1990. The letter also stated that the offer would be kept open until July 1, 1990. About two weeks later, Roberta sent Emily another letter in which she revoked her offer. On June 15, after she had received both letters, Emily wrote to Roberta accepting her offer of March 1. Discuss in detail whether there is a valid contract between Roberta and Emily.