jim and mark are negotiating a contract. jim draws up and signs an offer to buy mark's pet, a big-tailed rat. jim draws up a contract to purchase the rat for $450. jim delivers the offer to mark on may 30, 2016. on june 1, 2016, mark signs the contract and places it in his u.s. postal service mailbox. on june 2, 2016, jim emails mark and properly revokes his offer. on june 3, 2016, jim received the signed contract from mark in the mail. is there a valid contract?