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 2 or ball 3 is drawn
and the X's are zero otherwise. Show that any two of the random variables X1, X2, and X3 are independent, but the three together are not.