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Consider the undirected weighted graph G with 3 vertices, whose adjacency matrix is given as :

\(G = \begin{bmatrix} 0 & 2 & 2 \\ 2 & 0 & 2 \\ 2 & 2 & 0 \end{bmatrix}\)

Which of the following is true ?

  • A.Graph G has no minimum spanning tree
  • B.Graph G has a unique minimum spanning tree of cost 4
  • C.Graph G has 3 distinct minimum spanning trees, each of cost 4
  • D.Graph H has 3 spanning trees of different costs

Answer: C

Graph G has 3 distinct minimum spanning trees, each of cost 4.

The matrix describes a triangle on three vertices with every edge of weight 2. A spanning tree on three vertices uses 2 of the 3 edges, and there are 3 ways to leave one edge out. Each tree costs 2 + 2 = 4, so all three are minimum spanning trees.

A connected graph always has a minimum spanning tree, the MST is unique only when ties do not allow alternatives, and the spanning trees here all have the same cost (and option D speaks of a graph H that is not given).

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