S is a subset within a universal set, U. If S = {x, y, 4, 9, ?}, which could describe U? U = {keys on a keyboard} U = {letters} U = {numbers} U = {punctuation marks}

If the angle bisectors of a pair of opposite angles of a quadrilateral are the opposite sides of a parallelogram formed by the two angle bisectors and two sides of the quadrilateral, is the quadrilateral always a parallelogram? explain your answer.