Show that the following graphs are perfect:
(a) bipartite graphs;
(b) interval graphs: ({v1,...,vn},{{vi, vj} : i = j, [ai, bi] ∩ [a j, b j] = ∅}),
where [a1, b1],...,[an, bn] is a set of closed intervals;
(c) chordal graphs (see Exercise 38 of Chapter 8).