hi I have a math question about graph theory - Tree. If you regard the capital letters "A, E, F, H, I, K, L, M, N, T, V, X, Y, Z" to be graphs, which of them is NOT a Tree?

A graph consists of two finite sets, V and E. Each element of V is called a vertex
(plural vertices). The elements of E, called edges, are unordered pairs of vertices.
For instance, the set V might be {a, b, c, d, e, f, g, h}, and E might be {{a, d},
{a,e},{b,c},{b,e},{b,g},{c,f},{d,f},{d,g},{g,h}}.Together,V andE
are a graph G.

Graphs have natural visual representations. Look at the diagram in Figure 1.2.
Notice that each element of V is represented by a small circle and that each ele-
ment of E is represented by a line drawn between the corresponding two elements
of V .