Which of the following assignments will compile without errors? Explain your answers.
(a) RoundBalloon b1 = new RoundBalloon();
(b) Balloon b2 = new RoundBalloon(100, 100, 10, Color.RED);
(c) Balloon b3 = new SquareBalloon();
(d) RoundBalloon b5 = new Balloon();