Mike has a MasterCard with an annual fee of $25, an 18% interest, and a $1,000 credit limit. He always pays the total outstanding balance monthly. His most recent monthly statement lists last month's payment, new charges this month totaling $1,500, and a $30 fee. The fee is most likely the result of:
a. ​his annual fee.
b. ​transaction fees on purchases.
c. ​a late payment.
d. ​an over-the-limit fee.
e. ​interest charges.