Which CREATE CONSTRAINT command is correct?

A.
CREATE CONSTRAINT con1 ON mytable CHECK salary > AVG(salary);

B.
CREATE CONSTRAINT con1 ON mytable CHECK birth_date < :CURRENT_DATE;

C.
CREATE CONSTRAINT con1 ON mytable CHECK salary > (SELECT AVG(salary) FROM
corpsal);

D.
CREATE CONSTRAINT con1 ON mytable CHECK birth_date < EXTEND(CURRENT, YEAR
TO DAY);