If a loopless weighted graph (G,W) has a unique edge $\displaystyle e_{m} $ of minimum weight-- that is, $\displaystyle W(e_{m}) < W(e) $ for $\displaystyle e \neq e_{m} $ -- show that any minimum cost spanning tree of G must contain $\displaystyle e_{m} $.

I realize this is true, but I need to prove it. I used Kruskal's Algorithm to show that there is at least one minimal cost spanning tree that includes $\displaystyle e_{m} $. I'm just stuck trying to think of how to prove that edge is in ANY minimal cost spanning tree.