Which of the following queries contains an equality join?

a$.$ SELECT title, authorid

FROM books, bookauthor

WHERE books.isbn $=$ bookauthor.isbn

AND retail $>20$;

b$.$ SELECT title, name

FROM books CROSS JOIN publisher;

c$.$ SELECT title, gift

FROM books, promotion

WHERE retail $>=$ minretail

AND retail $<=$ maxretail;

d$.$ none of the above