Juliana offers in writing to sell her book collection to Linda for $15,000. Three days later, before Linda accepts, Juliana dies and her heirs tell Linda that the $15,000 offer is revoked and that they will not sell the collection for less than $20,000. One week after the original offer was made by Juliana, Linda sends a signed acceptance of Juliana's $15,000 offer. Has the contract been formed? Explain.