[Self-reducibility] Show how the optimization version of vertex cover problem is polynomial time-reducible to its decision version. The optimization version, known as min vertex cover problem, is this -- given a graph G, find the minimum set of vertices that cover all the edges in G. The decision version is this -- given a graph G and a constant k, is there a vertex cover of at most k vertices? Here, we say a vertex covers an edge if and only if the vertex is an end-node of the edge.