Which of the following is not a valid SELECT statement?
a. SELECT STDDEV (retail)

FROM books;

b. SELECT AVG (SUM (retail))

FROM orders
NATURAL JOIN orderitems NATURAL JOIN books
GROUP BY customer#;

c. SELECT order#, TO_CHAR (SUM (retail), '999.99')

FROM orderitems JOIN books USING (isbn)
GROUP BY order#;

d. SELECT title, VARIANCE (retail-cost)

FROM books
GROUP BY pubid;