Let \(G(V, E)\) be a simple, undirected graph. A vertex cover of \(G\) is a subset \(V' \subseteq V\) such that for every \((u, v) \in E\), \(u \in V'\) or \(v \in V'\). Let the size of the smallest vertex cover in \(G\) be \(k\). Let \(S\) be any vertex cover of size \(k\).
For a vertex \(v \in V\), which of the following constraints will always ensure that \(v \in S\)?