Which of the graphs described below must be a tree?
◦ A roadmap illustrating all of the towns and roads connecting them in Madison county.
◦ An ancestry graph: Sets of parents are the vertices; birthparents are joined to their single-child offspring by edges.
◦ The students at Tasmania State University are the vertices; edges represent whether two students have had a class together or not.
◦ all of these graphs must be trees.
◦ none of these graphs must be trees.