问题7： 两个block的“边界”是什么？ Corollary 5.9 Fvery two distinct blocks B and B in a nontrivial connected graph G have the following properties: (a) The blocks B and B are edge-disjoint. (b)The blocks B and B have at most one vertex in common. (c)If B and B,have a v vertex v in common, then v is a cut-vertex of G.Corollary 5.9 Every two distinct blocks B1 and B2 in a nontrivial connected graph G have the following properties: (a) The blocks B1 and B2 are edge-disjoint. (b) The blocks B1 and B2 have at most one vertex in common. (c) If B1 and B2 have a vertex v in common, then v is a cut-vertex of G