How many undirected graphs which are not necessarily connected can be formed out of a provided set $\mathrm{V}=\{\mathrm{V} 1, \mathrm{~V} 2, \ldots \mathrm{~V} \mathbf{n}\}$ of n vertices?
A. $n(n-1) / 2$
B. $2^{\wedge} \mathrm{n}$
C. n!
D. $\mathbf{2}^{\wedge}(\mathrm{n}(\mathrm{n}-1) / 2)$