Set B is the 2x2: infer the relation between the first pair and apply it to the second. The relation can be a turn, a flip, a move, a change of count or size or fill, a part added or removed, or an enclosure gained. Early items change one thing; later items change two together, or use a relation that only works one way round.
What you see: a pair that changes, and a second pair to finish. What you do: say the relation in a sentence and apply it.