DEFINITION: A set of vertices $A\subseteq V$ in a graph $G(V,E)$ is called a module if it satisfies the following property:
For every $v\in V\setminus A$, either $A\subseteq N(v)$ or $A\cap N(v)=\emptyset$, where $N(v)$ is the neighborhood of $v$.
Input: An undirected graph $G(V,E)$ and a natural number $k$
Output: YES if $G$ has a module $A\subseteq V$ with $|A|=k$, otherwise NO
What is the complexity of GRAPH-MODULE problem?