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Which of the following graphs are trees? (see figure)

Figure

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Correct answer: A. (A) and (B) only

Explanation

A tree on 6 vertices is connected with exactly 5 edges and no cycle. Graphs (A) and (B) each have 5 edges and connect all six vertices. (C) contains a cycle, and (D) has only 4 edges so it is disconnected.

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