Let A = [(1 2), (3 4)] and B = [(a 0), (0 b)]; a, b ∈ N. Then,
(a) there exists more than one but finite number of B's such that AB = BA
(b) there exists exactly one B such that AB = BA
(c) there exist infinitely many B's such that AB = BA
(d) there cannot exist any B such that AB = BA
