(7)_____ a book has been 'caught', the person finding it is encouraged to record the event by logging on to the book-crossing website and entering the book's ID number written on the label.
(7)_____ a book has been 'caught', the person finding it is encouraged to record the event by logging on to the book-crossing website and entering the book's ID number written on the label.
A. Soon
B. Since
C. Once
D. While