SELECT PRIMARY KEY

For data kept by a single publisher $--$ it contains an entity BOOK, the following is known about the four attributes identified as candidate keys:

BookName varchar$(50),$ is always known for each book and no two books have the same book name

ISBN char$(13),$ is unique and always known including for books in process of being published.

REFNumber char$(10)$ is an internal $10$ digit code assigned to every book but is common for different editions of the same book.

PubDate is the date on which the book is published. The publisher, never publishes two books on same date. Prior to publishing, every book still has a value which reflects the tentative publish date, and this is often revised frequently through out the publication process.

Based on the description above, the best suited attribute for primary key is: