Which of the following SQL statements is not valid?

a. SELECT b.isbn, p.name

FROM books b NATURAL JOIN publisher p;

b. SELECT isbn, name

FROM books b, publisher p

WHERE b.pubid = p.pubid;

c. SELECT isbn, name

FROM books b JOIN publisher p

ON b.pubid = p.pubid;

d. SELECT isbn, name

FROM books JOIN publisher

USING (pubid);

e. None—all the above are valid SQL statements.