B wishes to purchase a house from S for $400,000. B sends a written offer to S, dated 12/1, which specifies all the essential terms and which has a space for S's signature at the bottom. Above the signature line is the phrase: "There is no contract unless this document is signed by the seller." S decides to accept the offer. Although S does not sign B's form, she writes B the following signed letter on her personal stationery: "Dear B: I'm delighted to accept your offer of Dec. 1." S immediately mails this letter to B. Shortly after the letter is mailed (and before it is received), B calls S and attempts to revoke the offer. If S sues B for specific performance, the most likely result is: