19. Given the following SQL statement, which statement is most accurate? SELECT customer# FROM customers JOIN orders USING(customer#) WHERE shipdate-orderdate IN (SELECT MAX(shipdate-orderdate) FROM orders WHERE shipdate IS NULL); a. The SELECT statement fails and returns an Oracle error message. b. The outer query displays no rows in its results because the subquery passes a NULL value c. The customer number is displayed for customers whose orders haven't yet shipped. d. The customer numbers of all customers who haven't placed an order are displayed