7. For each of the following, determine whether it is valid or invalid. If valid then give a proof. If invalid then give a counter example. (a) (An B) u (Cn D)(An D)u (CnB) (b) A- (BuC)- (A-B)n (A-C) (c) BnCSA(C- A)n (B- A) is empty (d) (Au B) (An B)-B is empty