PL) 7. A box contains four balls, numbered 1 through 4. One ball is selected at random from this box. Let X1 = 1 if ball 1 or ball 2 is drawn, X2 = 1 if ball 1 or ball 3 is drawn, X3 = 1 if ball I or ball 4 is drawn. The X, values are zero otherwise. Show that any two of the random variables X1, X2, and X's are inde- pendent but that the three together are not