Help please!

Relation R has schema: CREATE TABLE R ( a INT PRIMARY KEY, b INT DEFAULT 0, c INT NOT NULL ); R is currently empty. Develop a test that determines whether an insertion into R is currently legal. Then apply your test to determine which of the following INSERT statements is allowable.

		a)	INSERT INTO R(c) VALUES(0);
		b)	INSERT INTO R(a,b) VALUES(1,2);
		c)	INSERT INTO R(c,a,b) VALUES(3,4,5);
		d)	INSERT INTO R(b,c) VALUES(3,4);