The trigger below is defined on the relation T(C, D) with the primary key on the attribute C. CREATE TRIGGER Secret AFTER DELETE ON T REFERENCING OLD ROW AS oldie FOR EACH ROW WHEN (oldie.C = ANY...

Extracted text: The trigger below is defined on the relation T(C, D) with the primary key on the attribute C. CREATE TRIGGER Secret AFTER DELETE ON T REFERENCING OLD ROW AS oldie FOR EACH ROW WHEN (oldie.C = ANY (SELECT D FROM T)) INSERT INTO T VALUES (oldie.C, oldie.D): Which of the following best explains the conditions as enforced by this trigger? a. Attribute D in T is a foreign key, referencing C. b. Every key value also appears in a non-key column. C. A key value that is present in a non-key column in another row cannot be removed. d. Attribute D is unique.