Which of the following SQL statements is invalid and returns an error message?
a. ALTER TABLE books ADD CONSTRAINT books_pubid_uk UNIQUE (pubid);
b. ALTER TABLE books ADD CONSTRAINT books_pubid_pk PRIMARY KEY (pubid);
c. ALTER TABLE books ADD CONSTRAINT books_pubid_nn NOT NULL (pubid);
d. ALTER TABLE books ADD CONSTRAINT books_pubid_fk FOREIGN KEY (pubid) REFERENCES publisher (pubid);
e. All of the above statements are invalid.