Theorem: If w,x,y, and z are integers where w divides x and y divides z, then wy divides xz. For each "proof" of the theorem, explain where the proof uses invalid reasoning or skips essential steps. (a) Proof. Let w,x,y,z be integers such that w divides x and y divides z. Since, by assumption, w divides x, then x=kw for