Consider an undirected graph G with six vertices labeled from 'a' to 'f. Its adjacency matrix representation is as follows: a b (c, e) (e, f) (b, c) (a, e) C d e f a c d e f 03 02 4 0 3 0 4 0 20 0 4 0 0 4 2 20 0 0 3 0 4 2 4 3 30 04 0 0 04 4 0 b 20 Which of the following edges cannot be in any minimum spanning tree of G?