Optimize the following Boolean functions by finding all prime implicants and essential prime implicants and applying the selection rule:
(a) F(A, B, C, D) Σm (1,5, 6, 7, 11, 12, 13, 15)
(b) F(W, X, Y, Z) Em (0,1,2,3,4,5,10, 11, 13, 15)
(c) F(W, X, Y, Z) Em (0,1,2,5,7,8, 10, 12, 14, 15)