An urn contains 12 balls, of which 4 are white. Three players $-A, B,$ and $C-$ successively draw from the urn, $A$ first, then $B$, then $C$, then $A$, and so on. The winner is the first one to draw a white ball. Find the probability of winning for each player if
(a) each ball is replaced after it is drawn;
(b) the balls that are withdrawn are not replaced.