Xiao Chen was refused by the library staff to enter the ancient books library because he did not have the status of a graduate student. Xiao Chen presented the ancient books good book library pass and said: According to regulations, I can enter the ancient books library. The rules of the ancient books library are: not graduate students cannot enter the ancient books library: Only if you have a pass to the ancient books, you can enter the ancient books. Xiao Chen is most likely to understand the provisions of the ancient books library as--. ['People who are allowed to enter the ancient books library may not be allowed to enter the ancient books library.', 'With the Passbook of Ancient Books.', 'You can enter the ancient books library if you have a pass for the ancient books.', 'You cannot enter the ancient books library unless you have a pass to the ancient books library.']