This question is based on successful execution of the following statement:

CREATE VIEW changeaddress

AS SELECT customer#, lastname, firstname, order#,

shipstreet, shipcity, shipstate, shipzip

FROM customers JOIN orders USING (customer#)

WHERE shipdate IS NULL

WITH CHECK OPTION;

Which of the following columns serves as the primary key for the CHANGEADDRESS view?

a. Customer#

b. Lastname

c. Firstname

d. Order#

e. Shipstreet