Donald buys a subscription to a national magazine that gives him two years for the price of one year. After the two years expire, Donald continues to receive the magazine for six more months, although he has not subscribed again. The magazine then sends Donald a bill for the magazines he received after the expiration of his subscription. Donald:__________.a. owes nothing.b. can be fined for failing to return the magazines.c. owes half of the bill.d. owes the full amount.