Seven balls are randomly withdrawn from an urn that contains 12 red, 16 blue, and 18 green balls. Find the probability that
(a) 3 red, 2 blue, and 2 green balls are withdrawn;
(b) At least 2 red balls are withdrawn;
(c) All withdrawn balls are the same color;
(d) Either exactly 3 red balls or exactly 3 blue balls are withdrawn.