单选题 College librarian: Until three years ago, nonstudents used the library without charge. Then, because of our decreased budget, they were asked to pay $100 per year. Nevertheless, about 150 nonstudents still use the library without paying. We will therefore gain financially if we hire a guard to identify nonstudents and ensure that they have paid the fee.
Which of the following would it be most important to know in order to evaluate the librarian's conclusion?

A、 The number of students who use the library annually.
B、 The library's budget this year.
C、 Whether the library has installed an expensive computerized cataloging system.
D、 How much the library's budget was decreased three years ago.
E、 (E) The annual cost to the library of hiring a guard.

【正确答案】 E

【答案解析】