Which of the following is a valid SQL command? a. ALTER TABLE books ADD CONSTRAINT UNIQUE (pubid);

Question:

Which of the following is a valid SQL command?
a. ALTER TABLE books ADD CONSTRAINT UNIQUE (pubid);
b. ALTER TABLE books ADD CONSTRAINT PRIMARY KEY (pubid);
c. ALTER TABLE books MODIFY (pubid CONSTRAINT NOT NULL);
d. ALTER TABLE books ADD FOREIGN KEY CONSTRAINT (pubid) REFERENCES publisher (pubid);
e. None of the above commands is valid.