The graph below represents a library fine, in dollars, for a book returned late after a number of days overdue. A graph titled Library Fines. The horizontal axis shows days overdue and the vertical axis shows the fine (dollars). The line shows an increase to $10 at 19 days overdue, then constant.

Christen returned an overdue book to the library. Her fine was the same as the amount on the following day. Out of the fines show below, which one would be closet to the number of days overdue?

10
14
18
21