Which of the following is a valid SELECT statement?
a. SELECT AVG (retail-cost)

FROM books
GROUP BY category;

b. SELECT category, AVG (retail-cost)

FROM books;

c. SELECT category, AVG (retail-cost)

FROM books
WHERE AVG (retail-cost) > 8.56
GROUP BY category;

d. SELECT category, AVG (retail-cost) Profit

FROM books
GROUP BY category
HAVING profit > 8.56;