Justin owned four speedboats named Porpoise, Priscilla, Providence, and Prudence. On April 2, Justin made written offers to sell the four boats in the order named for $4,200 each to Charles, Diane, Edward, and Fran, respectively, allowing ten days for acceptance. In which, if any, of the following four situations was a contract formed?

a. Five days later, Charles received notice from Justin that he had contracted to sell Porpoise to Mark. The next day, April 8, Charles notified Justin that he accepted Justin's offer.

b. On the third day, April 5, Diane mailed a rejection to Justin that reached Justin on the morning of the fifth day. At 10 A.M. on the fourth day, Diane sent an acceptance by telegram to Justin, who received it at noon the same day.

c. Edward, on April 3, replied that he was interested in buying Providence but declared the price appeared slightly excessive and wondered if, perhaps, Justin would be willing to sell the boat for $3,900. Five days later, having received no reply from Justin, Edward accepted Justin's offer by letter, and enclosed a certified check for $4,200.

d. Fran was accidentally killed in an automobile accident on April 9. The following day, the executor of her estate mailed an acceptance of Justin's offer to Justin.