On May 1 Ralph Offers to Cure and Smoke Sam's

On May 1 Ralph offers to cure and smoke Sam's pork, and promises to keep the offer open until June 1. On May 3 Ralph mails Sam a letter revoking the offer. Sam receives and reads the letter on May 5 and responds to Ralph on May 6. Ralph's revocation of the offer became effective on

A) May 3.
B) May 5.
C) May 6.
D) no date because the offer is irrevocable.