Set B is where the 2x2 arrives. The top row shows one figure becoming another; the bottom row starts the same way and the missing cell must finish it the same way. Early items change one visible thing, later ones two, and the last items of the set are printed black on white as a bridge to the Standard test.
What you see: a pair that changes, and a second pair to finish. What you do: say the change in words, then apply it.