title: Proving NP-Completeness of a problem
body: <p>We are given a set A = {a<sub>1</sub>,a<sub>2</sub>,...,a<sub>n</sub>}</p>
  
  <p>Given subsets of A named B<sub>1</sub>,B<sub>2</sub>, ..., B<sub>m</sub>. If a subset of A named H has intersection with all given B's, we call H "Covering subset". Is there any "covering subset" of size K (cardinality of H is K) for given A and Bs?  Prove that this problem is NP-Complete. </p>
  
  <p>We should reduce some known problem to "covering subset" problem.</p>