A subset S of vertices in an undirected graph G is called triangle-free if for every triple of vertices in S, at least one of the three edges (u,v), (u,w), or (v,w) is absent from G. The problem Large Triangle-Free Subset (LTFS) takes as input a graph G and a number k, and outputs whether G has a triangle-free subset of size k. Prove that LTFS is NP-Complete.