Simplify as sum-of-products for each function using K-map. Indicate the essential prime in the maps if any. (Note the dont-care notation here is different than in the textbook.)

(a) f(a, b, c, d) = m(0, 1, 3, 5, 6, 7, 11, 12, 14)

(b) f(a, b, c, d) = M(1, 9, 11, 12, 14)

(c) f(a, b, c, d) = M(5, 7, 13, 14, 15) D(1, 2, 3, 9)