A database table with three fields (bookname, author, language) has been created. If the table is as provided below, which of these three fields may be used as the primary key?
A Tale of Two Cities, Charles Dickens, English
Oliver Twist, Charles Dickens, English
Godaan, Premchand, Hindi
Chandrakanta, Devaki Nandan Khatri, Hindi
Hamlet, William Shakespeare, English
The Merchant of Venice, William Shakespeare, English
Only fill in the name of the field which may be used as the primary key. Grading is case-sensitive.