NP and Computational Intractability Question #5

Consider a set A -[aj,... , an] and a collection B,B2, A (i.e., B, S A for each i) , Bm of subsets of We say that a set H CA is a hitting set for the collection B,B2,. . . , Bm if H contains at least one element from each B-that is, if H nB is not empty for each i (so H "hits" all the sets B). We now define the Hitting Set Problem as follows. We are given a set {al, . . . , an), a collection B1, B2. . . . , Bm of subsets of A, and a number A