一个具有n（n＞0）个顶点的连通无向图至少有（）条边。

A．(B, F, G, J, A, D, I, E, H, C)
B．(B, F, G, J, A, E, D, I, C, H)
C．(A, B, D, C, E, E, I, J, G, H)
D．(C, B, D, A, E, F, I, G, J, H)

A．2,4,6,8,10,12,16,18,20,28,30
B．6,2,10,4,8,12,28,30,20,16,18
C．12,2,10,20,6,18,4,16,30,8,28
D．30,10,20,12,2,4,16,6,8,28,18