Though list B contains 4-3, so either B or D would be valid as ordering does not matter

When I first solved it I selected B but when I sat down to type it down I spotted 3-4 and thought that was the one I had gone with originally.

But in the solution they motivate the specific selection of B by stating that in the provided lists of pairs, the pairings are all with the greater number first.

Hence actually what I typed above is incorrect and the correct answer is B.

Sorry about that :(