At a movie theater box office, all tickets are sequentially prenumbered. At the end of each day, the beginning ticket number is subtracted from the ending number to calculate the number of tickets sold. Then, ticket stubs collected at the theater entrance are counted and compared with the number of tickets sold. Which of the following situations does this control detect?
a. Some customers presented tickets purchased on a previous day when there wasn't a ticket taker at the theater entrance (so the tickets didn't get torn.)
b. A group of kids snuck into the theater through a back door when customers left after a show.
c. The box office cashier accidentally gives too much change to a customer.
d. The ticket taker admits his friends without tickets.